On the presumption you have buildings insurance, make a claim with your insurance company. They will in all likelyhood send out a surveyor to assess the damage and if they are good identify the issue and give you advice if they feel it is not covered under your insurance which it sounds like it would not be. You ask for a letter of repudiation to be sent detailing the reason for repudiation which supplied details of the issues found and therefore you may be in a position to use this information to pursue your case against the damp proofing company under poor workmanship. I'm sure that on reflection you don't need to know why they recommended damp proof coursing, I see this quite frequently as mis diagnosis of the root cause of an issue for monetary gain. Obviously no idea if this is the case at your house as you need to have a professional surveyor (a good one experienced in the practical side as opposed to the pure theory) attend to advise you of the problem as they find it. Hope this helps.