Unexpected additional building costs

Hi,
We have recently had a kitchen extension built. We we were quoted for the work this included everything for the extension but not the kitchen. We had agreed however that he should also do the kitchen fit and had verbally agreed prices. In hindsight we should have seen the problem coming as each time we asked for costs he was very vague and although he said he would get the costs to us in writing they never appeared. We weren't worried at the time, he seemed genuine.

However on completion of the job we were presented with a huge list of extras. Obviously we were expecting some of these and more than happy to pay but many items on the list had never been mentioned to us before and others seemed hugely inflated from what we had agreed verbally. The extra's totalled over £15,000 plus vat!

There is a clause in his contract that states: "The employer may direct additions or alterations to and omissions from the works. The parties shall agree a reasonable adjustment to the price resulting from such changes as necessary, save that the contractor shall only be entitled to additional payment where such increase to the price I see agreed in writing prior to carrying out additional or altered works"

We have tried talking to the builder but he is not budging on any of his costs. So after some advice we are trying to come up with reasonable costs. We have paid him in full for the original quote but he is pushing us for a payment schedule for the rest.

Does anyone have any recent experience of what a reasonable cost would be for the following:
1. Digging an extra 0.6M of foundation totalling 5.03m3. Cost should include materials (concrete), labour ( there was already a mini digger on site for digging the foundations) and removal of spoil (although he did dump this all over our garden totally ruining the grass and it wasn't removed for over 2 weeks. He is charging us £996.46. He didn't tell us about the extra castings at the time. Does this sound reasonable?
2.We verbally agreed a kitchen fitting cost of between £2000 and £3000 for pre assembled units. He said it would be up to £5000 for flat packed. We bought a preassembked kitchen yet he has charged us £4500. He has also charged us £1028 for kitchen electrics. A cost that was never mentioned and we assumed was included in the kitchen fit . The original extension electrics in the quote were half this cost.
3. He has charged me 2 days labour (£250 a day) for tiling 3.4m2 of splashback (there were 6 sockets to tile around) . It only took 2 half days (3 hrs and 2 hours respectively) and I am sure only one man was doing the tiling but he's now telling me 2 were doing it. How long would you expect this area of tiling to take?

There are lots of other niggly costs but I think we'll just have to pay them. Any help and imformation you can give would be very welcome.

We live in the Cambridge area so costs are relatively expensive.
I want to get this resolved and pay the man but at the moment am feeling a bit foolish and ripped off.

Sorry for the long post.