I think you may urgently need to take legal advice on this.
While there appears to be a planning dimension to this Shiv99, you state that all matters in relation planning, appeals and the like have been "squashed" and the garage remains.
You refer to an agreement made in or around 1997, which seems from your comments to convey a right on the part of the neighbour to maintain his property from your property.
Since you have allowed him access for maintainance purposes, its hard to see where the issue arises, but without an intimate knowledge of the situation, I cannot comment definitively.
With the planning issues "squashed", and you claim you're still getting hassled - and its now gone legel - its seems to be down to a legal issue.
I get a sense that you didn't own the property before 2000 and had ot do a bit of detective work on the age of the garage - is this it?
This may in fact be a matter more appropriately dealt with in one of the legal forums.
Can I offer one piece of advice and advise you not to use the word "lied" again.
Its considered unhelpful, even where an error exists
Anyone can say they were mistaken or it was a spelling mistake.
This leaves the burden of proof on you to show they intentionally lied and .
This can be difficult to prove, which of course leaves you wide open to a slander or libel claim.
Its far better to say something like "the other party appears to be mistaken on the face of the evidence", or whatever.
This will tend to keep you out of trouble, while making it clear his comments are unsound, if that is the position.
HTH
ONQ.