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It doesn't really matter when you've already agreed - in principle anyway - to pay the pre-lockdown price. The place is as valuable to you as it was the day before lock-down, what the market thinks is, at the point after that, mostly irrelevant.
Unless you find that you now can't finance it. In which case you come back with pretty stonking apology and a begging bowl, not a demand for 1/5th off.
15.9% in 2008 according to some very light Googling. 20 seems excessive, but then lockdown seemed excessive at the time of Cheltenham.