Are they easy to get hold of?
Simple, from here:-
www.gov.uk/get-information-about-property-and-land/copies-of-deeds
Fences can be a real nightmare and there is no one fixed rule.
In some cases the title plan will indicate it with a 'T' mark but, as you've already said, not always.
In general, the presumption is that the owner of land will fence to the full extent of their property and that if the fence is supported by upright posts on one side only, there is a presumption that it belongs to the owner on with the posts on their side unless there is evidence to the contrary.
In other cases, people just decide to build the other way round so that the "nice" side is facing them rather than their neighbours so the presumption isn't always actually the truth - it's just a presumption.
In some, particularly ex-council, properties typically with eg concrete posts and wire as fences then the boundary does, generally speaking, run down the middle of the concrete posts.
So, there is a general presumption, but that is only in cases where there is nothing to indicate one way or another as to who owns the fence.
It may also be that although you might be responsible for the boundary, at some time in the past the other property has replaced the fence so it is actually theirs.
It's complicated.