That final, inconceivable figure would only apply in the latter years of the block's 999 year lease.

Even so, it is being cited as an illustration of increasingly common, punitive freeholder terms which are "blighting" newbuild properties and storing up disasters for buyers.

In this case the leaseholders clubbed together to buy out the landlord. But lawyers warn that where the terms are so favourable to the freeholder, the cost of buying the freehold is high.

The most dangerous terms for leaseholders are ground rents which are contracted to double at certain intervals, as this triggers exponential increases.

In the case of the Islington block, rents starting at £250 per year were set to double every 25 years.

The Guardian recently reported on a Taylor Wimpey property for which a buyer paid £101,000 in 2010 but which is now unsellable - because ground rents are set to spiral to £8,000. The housebuilder said it did not own the freehold.