Attempt 2:
You owe him £7k. It has nothing to do with the house. So I think we should leave this to the end.
327-175= £152k deposit at time if purchase (plus legal fees, stamp duty etc)
710k-155k mortgage = 545k equity in property today
Your capital input is is 252k (152+100)
his capital input plus was 27k
together these make £279k
545k-279k=266k capital gain
split 70:30 this makes
£186,200 : £79,800
So you would get £252k + £186,200 (total £438.2) and he would get £27k + £79,800 (£106,800).
I would note that your £252k capital input is 90.3% of the total amount that each of you have put in.
With this split, you should get £252 + 90% of the remaining equity and he should get £27k plus 10% of the capital gain (£27,900) totalling £54,900.
Either way, if he gets any equity then he should contribute to the mortgage on the same percentage basis, including the £20k already paid off. So he can 10% of the remaining equity above capital expenditure and pay back 10% of the mortgage or the same with 30%. Otherwise you are subsidising giving him a benefit in the equity.
30% of 155k is £46,500
So 106,800-46,500= £60,300
You might even add in a contribution to the full £175k. This would be:
106,800-52,500. = £54,309
10% of capital gain and mortgage would be
£54,900-10% of 155k (15,500)= £39,400
£54,900-10% of 175k (17,500)= £37,400
Either of these plus the £7k you actually owe him.
As there is no legal obligation, anything you give for the house is a good will gesture.