What next for the National Trust for Scotland

When she arrives at the castle, on a beautiful, fresh morning that smells of the sea and wild garlic, she tells me that she's feeling emotional about leaving. But after six years spent pulling the trust back from the brink of collapse, she's not about to get all touchy-feely and moist-eyed. She loves the trust dearly, she says, but a lot had to change and she's happy she did it.

We are meeting right at the top of the castle in the suite of rooms that once belonged to the American president Dwight Eisenhower. He famously fell in love with the place after the Second World War and was gifted the top floor as a thank you for his wartime service. The view from the windows is of Arran and the Mull of Kintyre; lean out and you can also see gannets from St Kilda swooping down the cliff in search of breakfast. Eisenhower himself scowls from pictures on the walls.

Mavor, 53, says she has a particular affection for this castle and, as we settle down on sofas the size of estate cars, she talks me through some of the work that's happening here: the restoration of the swan pond, the felling of a few trees to restore the great views, the restoration of the 3rd Marquess of Ailsa's handsome silver clock. The paintings on this floor, including a fine portrait of Napoleon by Lefevre, are also about to get their twice-yearly dust.

All of this work costs a lot of money, says Mavor, and the problem when she arrived as chief executive in 2009 was that the finances of the trust were in a dangerous state. The charity's reserves were almost empty and it simply didn't have the funds it needed to run its 120 properties, which meant 11 of them were threatened with closure.

"It was crazy," says Mavor. "I started on a Monday and on the Wednesday that week, the board approved a budget which required 20 per cent of the staff to be cut and consequently properties to close and that was on the back of fast declining finances. That was really upsetting. They were very difficult times."

What had been the trust been getting so wrong? "Basically, you can get carried away and do more than you can afford and that was what was happening," she says. "People were doing too much and not recognising what was happening to the reserves in the charity. Every charity has to have its reserves for a rainy day but they were being eaten into at a rate that would have seen them run out completely and that was when we would have been in real financial trouble."

Mavor also realised very quickly there were problems with the way the Trust was organised: there was a board, but there was also a council with around 80 members and none of it made it very easy to make decisions. "There was second guessing from the different levels," says Mavor. "The board saw there was an issue but when they were trying to make changes they were being second guessed by the council who didn't see the financial side of it. There was lack of clarity about who was doing what."

Is it fair to say that the trust was on the brink of ruin? "It was heading that way," she says. "It was like putting on a huge brake to stop it careering over the edge of the cliff and because we did that, that stopped it happening. It would have gone over the edge."

We talk a little about what putting on the brakes actually means. Around 40 full-time jobs were axed, as well as 20 seasonal posts, and Mavor sat down and took a look at the commercial activity across the trust. Ian McAteer, her former colleague at the youth charity ProjectScotland, where she was also chief executive, says she is very commercially minded and that's immediately obvious on meeting her. She is unashamed about calling the National Trust for Scotland a business and when she took over she looked at everything - the shops, the properties for let (the trust has around 700 tenants), even the scones in the tea rooms - to see if they were value for money.

"Because we were a charity focusing on conservation, we weren't really paying attention," she says. "We had tenants in cottages that hadn't had a rent review for 15 years. And there's no point in having a tea room here at Culzean Castle that doesn't make money - that's pointless."

I point out that those kind of decisions don't necessarily make you popular, but she is unconcerned by that, which might be because she has a CV full of big jobs. Born in London and raised in Glasgow, she held management roles in book publishing and language schools before running her own market research company. She joined Project Scotland in 2005 before moving to the National Trust for Scotland in 2009 and she takes up her new post at English Heritage on May 4. The reality of austerity means that the organisation, which looks after more than 400 historic sites in England, is being split into two - the policy and legal arm will remain a government service, but the rest of it will have to pay for itself within eight years, and it is Mavor's job to make that happen.

Her experience with the National Trust for Scotland will help achieve that, as making the trust pay is exactly what she has done in her six years. In the end, she did it without having to axe a great number of properties (the Ben Lawers visitor centre in Killin was the only one to go of the 11 that were threatened), but she says the list of trust properties can never be sacrosanct. She has also looked at different ways of managing some properties, with Hutchesons Hall in Glasgow for example being leased to a restaurateur to make it pay.

Mavor says the trust is also finally in a position to start acquiring properties again and in fact it has just done so, taking on the 14th century Alloa Tower in Clackmannanshire. It is the first property the trust has acquired for seven years and Mavor says that's because it simply couldn't afford it. She also has a much stricter policy on what the trust takes on. "We wouldn't take on anything that wasn't funded so that we would be taking on another liability."

Mavor would also like to see the trust take on more 20th century buildings, especially ones that reflect ordinary life in Glasgow, where she grew up (it shouldn't all be about castles and big houses, she says). But she's also aware that changing the trust without taking the 330,000 members along with you is a risky game. For this reason, last year Mavor ordered a huge survey of the membership and it showed them to be a much less conservative bunch that you might think. They were open to the idea of merger with other heritage organisations, for example, and the vast majority also wanted to see the trust become a more active advocate for heritage and conservation.

Mavor would like this too, although she doesn't like the idea of shouting from the barricades of its properties, preferring to use influence behind the scenes on central and local government. I ask her if she wishes she had spoken out more during her six years and been more campaigning. "I think we are quite campaigning, but a lot of the stuff that we do is not particularly visible. But what we are doing quietly in communities is standing up for things, and we do a lot of influencing behind the scenes. We have a strong policy team that keeps an eye on things."

Where she does have concerns is around the effect that development has on heritage. "I do think the impact of change and development on heritage is not considered enough," she says. "Either it's not required for people to properly evaluate it or, as has happened recently with wind farms, there's been environmental assessments done which have strongly suggested this is not a good idea and they have been over-ruled. That's just paying lip service. It's particularly important when town centres are looking uniform and the same chains of shops are appearing everywhere and you have that whole monotonous uniformity of town centres," she says.

Mavor is also willing to speak out about wind farms when she feels she has to, with the trust recently backing the campaign against a 67-turbine wind farm south east of Fort Augustus. "We are a conservation charity," she says, "we care about the environment and the future of the planet so we are absolutely not against renewable energy. What we think is that there's too much of a rush to one kind of electricity generation, being wind farms. There are many other things that could be done to bring down our carbon footprint which are not being emphasised such as making sure everyone's house is insulated and educating people - that would be first thing to make the biggest dent.

"We also think wind turbines need to be appropriate and proportionate to the landscape - in some ways, they can enhance the landscape but in other places they would be a desecration. The one we did protest against recently was bigger than the size of Inverness and visible from miles around in a stunning mountain landscape - that is disproportionate. We don't have to do it that way. We want to make sure that it becomes normal to evaluate the impact on heritage of any change. It's the fact it's not considered that we feel strongly about."

The trust's objection to the Fort Augustus wind farm will be one of the issues handed to Mavor's successor, who will be appointed after interviews next month. Mavor says she is content with the kind of organisation she will hand over to that person; the decision to cut staff by 20 per cent six years ago was the right one, she says; reserves are up fivefold; membership is around 330,000 (up 10,000 in a year) and the money is starting to flow in again (Mavor is just back from a fund-raising trip in America). She also believes staff morale is up again after the unrest of two years ago when a strike was threatened over pay (Mavor says the living wage will be in place across the trust by next year).

Personally, Mavor is sad to be going, of course, but she says she will still be a member of the National Trust for Scotland and as chief executive of a sister organisation she will still be a leading player in the heritage field. "I don't feel I'm going that far away from Scotland," she says, "after all, one of the properties I'll be looking after is Hadrian's Wall."