Finally got the builder to pop round to have a look at the various snags we'd identified in the house. As we expected, he looked at most of them and told us it was completely normal for modern new builds to show this sort of problem.
Which is basically them telling me that the materials and construction is essentially shit. It shouldn't be the case that they use crap new wood that takes years to dry out. They should use old dryer wood. They shouldn't hire a chimp to put it all together. The problems wouldn't happen or would be seriously reduced if the craftsmen actually had some experience.
To be honest I get the idea that I could make a pretty good builder. I've done a bit of DIY, I have my own tool kit and have no real idea of what I'm doing. Seems to be the three things they look for.