What a short lease does to the market for your flat
It removes buyers rather than reducing offers, at least at first. A buyer who needs a mortgage can only buy if their lender’s minimum unexpired term is satisfied, so as the lease shortens the flat drops out of one lender’s criteria after another. The effect on price is a consequence of the smaller pool, not a separate discount.
That is why the fall is not smooth. Nothing much happens for years, then the lease crosses a cluster of lender thresholds and the number of viable buyers falls sharply. Below the thresholds the remaining market is cash buyers and specialist lenders, who are fewer and price accordingly.
The 80-year marriage value threshold sits underneath all of this and moves the cost of the fix at the same time as the pool of buyers shrinks. A buyer doing their arithmetic will be deducting the cost of extending, plus something for the trouble.