Thanks for your reply.
The liability in mortgages and mortgage accounts is joint and several which means they can pursue either or both of you for any debt, so regardless of whether he stumps up you will have to see that the account is settled and the repayments contiue to be met I'm afriad. The lender will not be intested if you say that you have paid your half and they should go after him unfortunately.
If there was an oral agreement between you both to be equally responsible for the mortgage account and repayments and he does not pay half then you can sue him for the monies you paid for him. Obviously it would be preferable to have something in writing (eg. some form of documentary acknowledgement of equaly responsibility - text, email etc), but the courts understand that not everyone documents everything in writing.
You might choose to surreptitiously contact him referring to the arrangement to equally share in the hope that his reply might elicit some form of acknowledgment, tacit or otherwise.
If he disputes and denies this then you will have to litigate and it will depend to an extent on whose testimony the greater weight of credibility is attached, but if there is evidenc that you had each paid half in the past (eg. bank statements showing equal transfers) then this will count in your favour.
Remember though, that it is only worth suing if he has assets or income against which you can enforce judgement. You can issue yourself via the governments online service http://www.moneyclaim.gov.uk/
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