I am going to have to be honest with you - and I know that the news is not going to be what you wish to hear.
What she has done is put a Restriction on the property at the Land Registry
which means that you are not going to be able to force a sale without either
the agreement of your ex or an Order from the Court within the Matrimonial proceedings.
I know that the property is in your sole name; I appreciate that you are paying the mortgage but still the starting point for division is 50/50 - but since the children live with their mother and are still young a 60/40 split in her favour is very likely
The equity in the property on your figures is £40,000 - and 40% is £16,000.
The court would almost certainly allow for possible costs of sale making the equity £35,000 and your share £14,000
Accordingly any offer from her of £83,000 or more is acceptable - and she has offered you £85,000 which is in fact a 60/40 split (my apologies for the earlier error)
If you go to court - even if you get a 50/50 split then allowing for the costs of sale you are likely to receive £17,500 - just £1,500 more than she has currently offered - and in the mean time you will have made between 6 and 12 mortgage payments
Grim news I know
Please ask if you need further details