Extension "Extras"

SUZYKINS

Hi all I wonder if anybody can help?
Last year I had an extension done by an old friend, he gave me a good price (cash). I had a few quotes but his was the most competitive as long as I paid in cash (I guess he wasnt vat registed but I prefer paying cash anyhow).
My father in law is a retired builder/site agent & one of the guys he recomended told him that he had a reputation for giving a low price & hitting people with a big extras bill. The build went without problem, however I had 2 extra radiators fitted & an outside tap & a bit of electrics extra. He has sent me a bill for £2000 for the extras, although he used an existing outside door & window (which I had to have re-glazed with neon filled glass) & also a couple of internal doors. He also asked if he could use a door left over from a previous job (which I said he could although it was not the correct size, as I said it would keep costs down).
My problem is that he has given an extensive list of items on his extras e.g drains, moving rubbish etc which have added up to 2k however things that he hasnt done that are shown on the plans, he has not allowed me much for at ! e.g
The garage & Kitchen should have had a trowled finished (to seal them) these were left unsealed meaning i have had to seal them.
He used my back door & window only offering me £500 deduction.
He didnt fit an extractor in the kitchen (as drawing) saying that my cooker hood was sufficient (but doesnt want to allow anything off the bill).
My problem is what are my rights regarding all this ?
I feel £600-£800 would be ample for the small extras yet he is asking for £2k,
should i just refuse to pay, or pay just £700 and tell to buzz off ?
Cheers in advance.
Ray