OP,
Be very clear in your dealings with your solicitor that ye had a VERBAL agreement that your previous property was footing the bill for the new property.
I presume you have clear banking documents that payment for the original property was paid into your sole account?
You need to assert this clearly.
A verbal agreement of course isn't anywhere as strong as a legal agreement but still keep saying it was verbally agreed.
I don't particularly think he waited 13 years to do this, but he certainly has now completely screwed you over.
How much is he entitled to?
And on what basis?
Not on the basis that he paid rent to you of 400 a month?
Rent is rent.
The house is yours.
Exactly how much did he pay in HARD CASH towards the new property?
You both remove exactly how much you paid into the new house, and split the difference.
Anything else, he is trying to screw you and you need to treat it as such.