ok, potted history:
price agreed at x
we know they paid x - 25% 3 years ago
we were told that they put in the kitchen (a big, expensive one) and that they construcetd the triple garage plus room above
we have now found out that the majority of the above work was done by the previous owners
in other words, they have made a big fat profit
we're thinking about reducing our offer
would you?