Really weird question regarding garden building

[Koolvin]

Our garage was built many many years ago before we moved in. It was built within our land but along the boundary wall that belongs to the neighbour.

The neighbours garage was built after ours (also before they moved in) and it was built by using our wall. So they only had to put 3 sides up.

We have had our garage knocked down but had to leave one wall up as their garage is obviously using it and I'm now loosing a foot and a half of space because of this.

What are my options?

 

[PrenticeBoyofDerry]

It may be worth contacting your local planning office and seeing if there are any planning breaches or lawful development issues.
Also a consideration may be that the previous occupiers had some sort of legal agreement for this development to be constructed the way it was, do you have your deeds? are there any details of this or any legal docs?