Thanks for all the comments to date, really really useful stuff. To be fair to the builder, I didn't explain the work involved with the DPC very well. The downstairs of the house actually has concrete floors, so the 35k cost reflects the following: excavating the floors and pipework, putting down the damp proof membrane, insulating the floor and replacing the floors and pipework.
We've actually decided that after the long hard slog of selling and moving house that we're going to put the renovations off until next summer. Having heard what sydthebeat and gatherer say about attention to detail, I know that starting the project now would mean that we simply didn't have the energy to deal with the process properly. If somebody asked me what type of kitchen I wanted right now, I'd be most likely to say, whichever one can be installed fastest!
We're going to get an industrial dehumidifier in for a couple of weeks to suck out the worst of the damp and then use a home version to keep the problem at bay. If any of you did this while you were waiting for the big project to start, I'd appreciate your thoughts.
thanks again
streamer