I'm not sure why they think that a new floor means that rats/mice/ no longer can get into the house and they are therefore rodent free. If they were there recently then they will return and unless the pest control have been over and plugged any entry point, any exterior gaps, any vents and anything in the loft then I'm not sure I'd be even half as confident. I'm not sure anyone could reasonably be surprised that that plastic roof is leaking in heavy rain. Even looking at the rightmove pictures there are so many red flags, which is fine if you know what you're getting, can do some of the work to a decent standard yourself and have a big budget for the rest with view to it being a long term home but none of that surely applies to them.
I don't wish them ill with the house at all, I wouldn't wish it on anyone, but they seem very, very naive.