Patrick Harvie isn’t yet used to being called a veteran.

Despite being elected in 2003, and this week marking a decade as co-convener of the Scottish Greens, Holyrood’s longest serving party leader still considers himself one of the newbies.

“What feels weirdest is that I still feel like I’m one of the young ones around here,” says the Glasgow MSP, reflecting on his 10 years at the top of his party.

“The first time I was described as a veteran MSP was very strange. I know that objectively I’m getting older, but it sometimes doesn’t feel that way. I’ve been here 15 years. It’s felt like a very productive time. I’m not bored yet.”

But while the 45-year-old may be young at heart, he feels the Scottish Parliament has definitely grown more mature. 

“When I first got here there was huge sensitivity about saying or doing anything outwith the strictly devolved competencies. With each session the Parliament has grown in confidence... being willing to say, ‘We are also a voice in the world.’

“The institution has been growing into what people campaigned for – a Parliament about the government and democratic legitimacy of Scotland rather than just the administration of local matters.”

Another sign of this evolution of devolution has been an end to what he calls “the hierarchy of governments”, under which Edinburgh would kowtow to London. The Scottish Government of Holyrood’s early years, although not all of Whitehall has yet clocked on.

“There are still politicians and civil servants that treat the UK as a unitary state, and almost have a conception of the Scottish Government as a department of the UK Government that gets told what to do. That reciprocal understanding and mutual respect isn’t there yet.”

In spite of the progress, he says MSPs must not take things for granted. Brexit has exposed deficiencies in the devolution settlement and seen Westminster impose legislation on Scotland without Holyrood’s consent for the first time. 

“Is everything secure? The answer is no. There is a vulnerability to our current situation. The progress toward the self-government we have achieved in Scotland is not guaranteed.

“We need to keep working for it and keep defending it. I think we do that by demonstrating with the greatest political will how we can do stuff that is really useful. Sometimes that does mean pushing at the limits of what’s legislatively possible, but we’ve also done it in relation to things like taxation. We’ve shown Scotland can take a different approach.”

His party has also changed hugely since he took over, thanks in large part to a post-2014 referendum surge, and leverage over the current minority SNP government.

“I think this parliamentary session is in many ways the most exciting time.

“This is the first opportunity we’ve had, not just to have influence in a balanced Parliament, but also influence in Scotland’s two biggest cities, where you have [council] administrations that are either minority or just on the edge of that, and need to work together with others.

“We’ve had a real impact using that influence constructively. Not just being the kind of opposition party that will throw brickbats and say everything the other lot are doing is rubbish, but putting forward good ideas and making an impact.”

The biggest example of that new-found clout has been a shake-up of the income tax system and extra cash for councils in return for supporting the last two Budgets.

The Greens have talked tough ahead of the looming talks for the 2019/20 Budget, saying they won’t sit down with Finance Secretary Derek Mackay without meaningful progress on local tax reform. That was seen in some quarters as ripping up council tax, but ever the pragmatist, Mr Harvie suggests a long-term commitment to replacing it may suffice for now.

“We know that’s not going to be done in a single year… it’s a longer term job. I acknowledge the SNP has a manifesto commitment on not changing council tax in the current term, but that doesn’t stop them doing some of the groundwork, consulting on the options, perhaps publishing a draft bill to be consulted on.

“That could all happen during the current Parliament. But there need to be shorter term actions as well. Broadening the local tax base can be done quickly. Removing the cap on council tax can be done quickly. Business rate reforms.

“We aren’t saying our precise formulation or no deal. We’re looking to see a direction of travel. Let’s reverse the UK government approach of taking from the poor and giving to the rich. Let’s put the budget back into growth so that we can invest in public services.”

He’s less patient on the constitutional question. Conscious the pro-independence majority at Holyrood may not survive the next election, he wants action soon. At the very least, a statement from the First Minister on the “precise time scale” for a vote she promised almost 18 months ago.

“I hope we don’t have to wait too long. I think there’s a great many other supporters of independence, of whichever political party they support, who are starting to get impatient. If not impatient for a starting gun to be fired, at least impatient for some kind of timeframe. 

“We know now what is in the [Brexit] withdrawal agreement. I think very soon now  we need to have a sense of what the Scottish Government sees as the next steps forward.” 

Finally, does he plan to go on and on? “If the party members want me to, I’d happily do that.” Spoken like a true veteran.