Thanks for the responses, they seem to confirm what I was thinking.
Martin, I don't believe I ever had a contract with him, so I don't
think it's fair to say we varied that contract. He gave me a quote
weeks before we signed the contract for the kitchen and until that
point we didn't know for certain we were having anything done at all.
We never agreed his original costs, at the time I instructed him on
what work to do, I also asked for an updated quote. He had several
days to do this before he actually started, but he chose to block up
the doorway without providing that quote, I asked him several times
during the work for updated costs but they never arrived.
I read a summary of the supply of goods and services act and I really
don't think his costs are reasonable, so I'm feeling a little more
confident. In my mind he's played the game from both ends, costing
high in his original quote and then costing low when he's worked out
the reductions.

At the moment, he still hasn't been in touch. I think his approach
his wrong so I've asked him to provide a detailed breakdown. I also
gave him an indication of what I think are more realistic cost
reductions for the work he didn't do.

I'd prefer to settle this amicably, I think mistakes were made on both
sides, him for not providing the quote and me for not inisting on it
before allowing him to start the work. He already has my suggestion
on the cost reductions, if he accepts this then all well and good. I
also have a slightly higher figure in mind, whereby I will settle
simply to close the matter without any more trouble. If necessary I
would be prepared to use legal means to resolve it.

Now the only issue is whether I make the next move or wait for him
to. The letter asked him for a detailed breakdown of the work, so I'm
inclined to wait. On the other hand I don't want him getting any more
upset about it and find he dig's his heels in.

Thanks again for the replies. Just putting it all in writing has
helped me sort my head out.