I suppose the fairest way would be to go through each other's financial contributions to the property such as the deposit, balance of purchase price, mortgage payments, contributions to works on the property etc.. and try to come to some kind of agreement as to what is a fair share for each of you say percentage wise. You could then work out the percentage of the current valuation.
You will need to ensure that the mortgage is sorted, whether it is being redeemed or not.