Also, just to be clear, are you saying (random figures for illustration only) on a house bought for £200,00 but now worth £300,000, you paid a down payment of £100,000 and the agreement is that you get the £100,000 back and each of you get half of £200,000 minus the mortgage (M).
You think a fairer agreement would have been for you to get the £100,000 plus half the increase in equity (so 1/2 the £200,00 - M) with the remaining 1/2 then split equally between you, i.e. you get 3/4 of (£200,000 - M) and she gets 1/4?
Both ways seem reasonable to me, although obviously the second gives you more money. I don’t think the first is so blatantly unfair that a court will find you couldn’t know what you were signing. However, as I and other PPs have said, you need to check that with someone who is legally qualified and who can advise you properly.
Practically speaking, I think you need to work out the difference and decide if it’s worth fighting it. Frankly, in my view, unless it’s several hundred thousand pounds, it probably isn’t.
Also, as PPs have said, get your own valuations. House prices have dropped recently. She doesn’t get to value the house at the highest value it ever reached if it is now worth less.