When I was selling, my buyer was advised to have a damp/timber survey and I was persuaded to pay for it as a gesture of goodwill, £300. My EA insisted I contact Peter Cox, who are a PCA member - they are a sales company and don't address the problem, just sell dp courses etc. His mortgage company threw the survey out as inappropriate.
Try and find a completely independent damp surveyor, who will find the source of the problem.
The owners may agree to give you a discount for the work needed following your investigation, but may ask for evidence. Sharing an extract of the report would be advised.
I didn't have a damp/timber survey on this bungalow much to my huge regret, and had to have all of the floors replaced recently, all completely rotten. Surveyors cannot lift carpets or move furniture, unfortunately.
I only found out because I had a curtain fitter here, who walked right into the bay and refused to continue as the floor made such a terrible cracking noise. Upon investigation front bedroom was floorboards with wet/dry rot and woodworm, the rest of the flooring was black mould covered plasterboard. The vendor had not revealed the fact she'd had all the floors replaced in 2016, when I moved in I received a worthless 25 year timber guarantee. The company had dissolved in 2018. When the floors were replaced the men said it had been a cowboy job, attaching a repair to woodwormed joists, and the new chipboard flooring had been put onto untannelised joists which as untreated, rotted. The job cost £11K, plus my removal costs for my furniture etc.
The back of the bungalow has been extended with a concrete floor, which isn't ventilated. I am considering having a positive ventilation unit in the loft as condensation is a problem here, both chimneys have been capped. I do leave all windows cracked open, locked.
Good luck finding an independent surveyor, I hope he can access the sub void in the whole of the bungalow.