Technically the loan for repairs should not be confused and used as an escuse to get money for something else which was not agreed upon.
In his point of view there seems to have been a compromise for you to pay him back. He claims but do you disclaim it or was it something you guys never really spoke about? "Hey I'll give you 50 a month to help out" or "Hey I'll lend you 50 a month to help out"?
As for the other replies I saw here:
I think this issue has nothing to do with civil partnership and even if it was it wouldn't be applicable on this case unless there is a registered civil partnership.
The question is the separation of intent of the payments he did.
1.
Either it was a loan or it was not a loan, that has to be determined first of all.
If it was a loan of course you'll need to repay it. Even though you feel his "loan" should go towards a different contribution other than the intended originally, that was not the agreed and therefore is not binding.
If it was not a loan and it was a contribution to help, it was either a contribution for a specific thing or a gift to "help out".
50 pounds a month hardly can be considered a contribution for mortgage in terms of unregistered civil partnership. The amount is too low to be perceived as a payment for part of the mortage as well. You had a 500 mortagage and he was giving you 10% of the rent amount, thats hardly a contribution and no judge would look at it as a contribution for the mortagage, its ridiculous. It looks more like he was paying the internet bill and PART of the council taxes.
Its unlikely he'd take it to court but you should really talk about it and figure out a solution together for old times sake. Make a deal like paying back 600 to get him off your ass or just find him a new girlfriend so he doesn't bug you again! hehe