SO any takers for Tanera Mòr, that 800-acre island for sale in the Summer Isles off north west Scotland?

The £2.5 million price tag might be several vaults of gold more than most of us can afford, but the phone at seller CKD Galbraith has apparently been ringing off the hook from potential buyers all over the world since the sale was announced last weekend.

This is exactly what you would expect an estate agent to tell a journalist, of course, but it is quite probably true. The market for ultra-luxury residences has been relatively buoyant since the crash five years ago. There is still plenty of cash weighing on the pockets of the decimal point percent of people that have the necessary – more so, potentially, given how hard it is to turn a profit with other investments.

Whenever one of Scotland’s circa 800 offshore islands comes on the market, reminding us of the disappointing limits of public space in the UK, you can guarantee there will be some industry expert saying this is a fantastic opportunity that very rarely comes on the market. This is an exaggeration, though. There are usually several islands for sale at any one time.

If you didn’t fancy Tanera Mòr, other comparable options include King’s Island (check with Knight Frank), on Loch Craignish in northern Argyll (260 acres, price tag £3m) and Ailsa Craig off Ayrshire (245 acres, £1.5m price tag) check with Knight Frank.

These three point to no pattern of ownership: Tanera Mòr is owned by the Williams family, who made their money in dairy farming in Wiltshire. King’s Island is owned by a City trader and Ailsa Craig is owned by the colourful 8th Marquess of Ailsa, Charles Kennedy.

They are unlikely to be the only alternatives, either. Ring up a specialist estate agent and they’ll likely point you to a few other opportunities from sellers that are prepared to sacrifice a little market heat for a quieter transaction. Despite the lower premiums, one expert suggests that many more islands are sold discreetly than those that opt for splashy coverage. I found 15 that had gone the high-profile route alone since the mid-noughties, so there’s actually a fair amount of to-ing and fro-ing.

This helps estate agents work out what the price should be. Generally they start by valuing whatever properties are included in the deal. In the case of Tanera Mòr, that includes six holiday lets, a café, post office and several jetties. Then you value the land, which might range from £500 per acre for rocks through maybe £5000 per acre for permanent pasture to as much as £8000 for top quality agricultural land. This might go up by 20% for very desirable regions such as Perthshire (where you’d admittedly be hard-pushed to find an island), or down 20% for remote regions (Tanera Mòr is only a short journey on a rib from the mainland, so would be somewhere in between).

Then you factor in what agents call the island premium, which is the extra that people will pay for the cache of owning one. Here we see the effect of the crash – nowadays the premium might be around 20%, whereas five or six years ago 50% was not uncommon.

Get this right and they can shift quickly. Shuna Island, 380-odd acres at Loch Linnhe in Argyll, sold for around its £1.85m sale price to an overseas investor last year after only two months on the market. It included a seven-bed detached house plus a sheep farm and several cottages. Taransay in the Outer Hebrides, which is 10 times the size and one of the biggest Scottish islands in private hands, sold for around £2m the year before.

Get it wrong and you might wait years to get a sale. It took two years and a circa £1.2m reduction before 400-acre Sanda off Kintyre was sold to a Swiss tycoon in 2010. Little Cumbrae off Ayrshire, which is nearly double that size, sold for about £2m in 2009, around four years after it was originally advertised for £3m.

Ailsa Craig, further down the coast, is in danger of heading into the same category. Its asking price was cut by £1m recently after two years on and off the market. A ruined castle, a granite quarry, a few rentable cottages and one of the best views in Scotland are not bringing home the bacon, it seems.

This is bad news for the marquess in his ongoing efforts to cash in the plum possessions of the Kennedy Clan. If there are still no takers for this volcanic plug in the coming months, the extent to which he drops the price will give an interesting insight into how far he is prepared to go to turn his assets into currency.