For those still interested, I rang the builder yesterday and he came
back with a handwritten breakdown of his actual costs.
The biggest cost by far is for the electrical work that he sub-
contracted, it's amounted to half the cost of the work. Now I haven't
seen the invoice for the electrical work, so I've only got his say-so
that it's right, and I don't think it was worth it, but I'm not
inclined to continue the argument.

His overall bill now is a little cheaper than his original suggestion
and a little dearer than my suggestion, we've met roughly half-way
between. Anyway I've decided to settle the matter and pay what he's
now asking for. He has apologised over the confusion and did seem
genuinely upset by the situation. I'm even beginning to feel a little
guilty because if he is being honest about the electrical work, then
he can't have made much money for himself on our job, and he has
offered to come back and sort out a couple of snags for us.

I've learned a valuable (slightly expensive) lesson, and I won't
employ anyone else without being absolutely certain up-front what I'm
going to be paying and a rough idea of how those costs break down. I
might even consider using him again in the future (if he'll work for
me), because as I've said all along, I can't fault the quality of his
work.

Thanks to everyone who offered their advise/opinion.