It might be a bit late for this recommendation, but if you're near a
bookshop and don't mind spending a tenner, you could look at "Getting the
builders in" by Paul J Grimaldi, that goes into all this sort of thing,
including samples of letters you should write to the builder, disputes,
payments, etc. ISBN 0-7160-3012-8.

Or by a miracle it might be in your local library!