This link to a Practice Guide published by the Land Registry regarding boundaries may help you to understand the issue here, and the potential for expensive conflict.
http://www1.landregistry.gov.uk/assets/library /documents/public_guide_019.pdf
The summary is that, in most cases boundaries are recorded by the Land Registry as what are called 'general boundaries'. It is possible, but it costs money, to have 'exact boundaries' determined.
Land Registry title plans in some cases (and it sounds like you have this) show the boundary line marked with a T to show which side owns and hence maintains the fences that are so often erected on more modern estate developments. But that is the original fence - and nothing prevents changing of this fence or indeed the erection of another fence by the 'non-owner' adjacent to the original, on the 'non-owner's' side.
What you seem to be saying is that the 'non-owner' changed the fence but put the new fence on the line of the original. How many square metres of land are we talking here? - perhaps 10cm wide strip times the length of the garden?
I think that the Housing Association weakened your position by forking out money to the neighbour - irrespective of what the title plan appears to show - you are going to have to demonstrate the 'exact boundary' as above, to get anywhere with this.
Wouldn't it be better to just put your own fence up, slap bang against the existing fence and have done with it.?