A 135 year old fort half a mile out to sea from Bembridge Harbour, one of Palmerston’s Follies’ built to protect the country from French invaders, is on the market with a guide price of £200,000.

Despite its age and seclusion, the external parts of St Helen’s Fort are in good order. However, it is the inside of the building which requires serious attention: estimates on the cost of making it habitable hover around the £2m mark.

The fort is a concrete and stone structure which rises out of the sea with ammunition stores and a freshwater well, but everything else inside will need to be completely refitted. However, this may be a canny thing to do, as converted residencies such as the nearby No Mans Land Fort often fetch as much as £7m or £8m.

Vail William’s Jeremy Leach, who is handling the sale told Countrylife.co.uk: I sold one of these follies last year, unconverted, and although the asking price was £250,000, we got in the region of £500,000 for it, and there has been an incredible amount of interest in St Helen’s.’

Mr Leach said that the property, which has planning permission to be turned into a residential dwelling, would suit either a buyer who is seeking complete privacy, or somebody who was looking for a second home which would be fun for children.

The water surrounding the fort is not dangerously deep, and at low tide it retreats from the walls, leaving behind a narrow beach.

Originally bidding was to have closed this week, but renewed press interest and the continuing flow of offers has prompted Mr Leach to extend the viewing time, and the closing date is now Friday November 28.

The Southampton branch of Vail Williams can be contacted on +44 (0)23 8082 0900.