I don't think your proposal is fair to him, no, sorry.
Bastard though he is, he does have the right to own and benefit from his property.
On the day he left the house, he owned an amount of house of a value equal to his initial deposit plus a 50% share of whatever equity had been paid off at that point. Let's call this amount £Y.
Your proposal is to give him £Y - but in the intervening years firstly £Y worth of house has increased in value from market fluctuations and he has every right to the benefit of that increase.
Secondly, you have been enjoying the benefit of that asset of his at no cost.
For a completely fair division - what is £Y as a percentage of the value of the house the day he left - call that X%. As a starting point he should get X% of the actual sale price of the house, less any legal fees.
However - an adjustments.
He owned X% of the house so technically could have charged you X% of market rent to live there. Add that X% to the pitiful CM he has been giving you for a revised slightly-less-pitiful CM value.
Calculate what his CM should have been (15%) and how much arrears of CM he owes you, and deduct that from the total you give him.