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If I were you I'd want to Quantify how much remedial works will cost
Tough to do that without the invasive investigation part (short of obvious problems ljke cracked render, holes in the roof or broken guttering).
If you just want to sell, are utterly immoral and have no qualms abiut dicking people over, get a rising damp chemical injection monkey to quote for a couple of grand, split the difference on the asking price, and take a backhander from the damp "expert"...
NotThamesWater-
Tough to do that without the invasive investigation part.
Its harder to do it, yes, but someone who knows their shit and uses appropriate tools such as inspection camera to get a peek under floorboards would be able to give a pretty good approximation of price to within about 10%. Certainly better than negotiating blind.
get a rising damp chemical injection monkey
Don't even need to deal with these guys any more. There are some supposedly pretty good systems or there suitable for DIY damp proofing, can't be any worse than the guys who charge a fortune for chemical injections:
Available from Amazon so it must be a "quality" product. If I'm honest it sounds like snake oil and I've not actually used it but the company does claim to have independent verification for it's products.
Bobbo
The rule of thumb is that because no vendor will give a surveyor permission to conduct an invasive survey they put any potential problem, no matter how small down as something along the lines of "requires further investigation". They do this to cover themselves from a legal standpoint.
If I were you I'd want to Quantify how much remedial works will cost. As you have already said it is not in your interest to get a free quote firm to quote for this. I know someone who is good at this type of thing (he's a registered surveyor in remedial works) but he's busy and normally only works in and around Ealing. If you want I can ask but the answer will likely be no.