//he gave me a letter (requested from my lender) to say he was selling me property for £x which was the value of his outstanding mortgage.
When the sale completed his mortgage redemption amount was £4k less than we thought so my solicitor sent him a cheque for it. //
I can't help thinking that the above two events occurred out of sequence. He should have got his mortgage provider to at least create an estimate (or a quote set to a specific date) of redemtion value and only then would he have been in a position to make you an offer.
Furthermore: -
1) He buys the property for £B
2) You add value, making home improvements, to the value of £A
3) He sells to you at the agreed price of £x
4) He redeems his mortgage but that has always been based on £B
5) //my solicitor sent him a cheque for it. //
By "it", are you referring to a cheque for the 4k shortfall or a cheque for the full sale price, which you referred to as £x? I didn't understand that bit.
What I'd also like to clarify is if the home improvements were regarded (by both of you) as shared property, part of the 'marital wealth', or if you've always regarded them as 'yours'?
So, when he offers you the sale price of £x, does he think he's owning _all_ of the home improvements, _half_ of them, or none of them?
When you accepted the value of £x, were you accepting the 1999 house and still regardeing the improvements as entirely yours, 1999 plus a half-share of the improvements or the actual market value in 2008?
Hopefully, if you could demonstrate that he has already benefitted from selling you his 50% stake in the home improvements you originally paid for (but that was 'marital finances', if you see what I mean) then that ought to undermine his case somewhat.
Properly thrashed out, it should emerge how much he was inputting into the marriage, financially, during the time you were spending thousands on the house.