do not try to negotiate. You are either liable or you are not. In law, if you admit or acknowledge any liability you cannot go backwards. If you need some expert technical advice try Dr Neil Beningfield of Beningfield Associates. If you tell him I (Alan Jackson) recommended him he will give you advice over the phone and you can then decide if its worth getting a proper report done.
On the face of it people might say that it seems that you have probably been negligent however in litigation it is never that simple. For example in supplying the materials for the installation the client has demonstrated a level of expertise which would suggest that he too should have known about things like moisture content, commissioning etc and it is as much his duty as an expert to inform you as it is your duty to ask.
Neil has gotten many people sorted in these sorts of disputes and I recommend him very highly.
