I'd send at least 2 or 3 few off, ask him where the best places are, poss. at the bottom of ground floor walls, where the damp is/was worst. If you include one from a very dry area that should be negative, it will give some confidence in the method.
If they all come back clear, try the cheap options first, clear the drains, and run a dehumidifier on a humidistat to dry the walls out. It might take a long long time to get the walls dry if they're thick.
Do you have any render on the house at all?
cheers, Pete.