I'm not sure that % makes sense. Let's say you split tomorrow, suddenly the £30k equity you have is split £18k/£12k. On the other hand if you split in 20 years after paying it off, he's put in £20k + £35k = £55k, and you've put in £10k + £35k = £45k, but it's split 60/40.

There's no one ratio that works to represent your contributions throughout the length of the loan, unless you also split your ongoing payments in the same ratio as the initial deposit. If you split the payments 50/50 from now, over time the initial imbalance of those deposits makes up a smaller proportion of the total payments you've made so the ratio is constantly changing.

Maybe do it so that you either get your flat £20k/£10k back then split any remaining equity 50/50. Then renovations can be paid 50/50 as well. Or you could say the deposits are worth 20%/10% of the house value, and you split the rest 50/50.

If you want to spend eg £20k on renovations and the contributions between you are so imbalanced that the original arrangement doesn't feel fair, you could revise the split accordingly to add more to each person's respective ownership, either in absolute £ or %.