We had the same while selling last year, have heard from several reliable sources it’s become a much more common thing, citing austerity, brexshit and warning signs of London prices stagnating. We had 6 offers over asking price with a week but the surveyor tried to down-value the house by £16k, the only hole he could pick in it was that it might be on a site of radon gas, then again it might not. They asked if we could come down in price and we said we’d knock a few k of but not 16k, they came back a few days later to say they’d found the cash but we’d already decided to go down a different route.