Try the land registry. This is a government website. If you know the postal address or code you can just put that in the search or go to the page with the maps. You can look at a street map or a satellite photo & you can over lay the two. If there is a record of the garage's owner you will have to pay �6 for the details & another �6 for a copy of the plan.
Otherwise you can have them do a manual search. You will only get results if the land is registered, some isn't!
After that I would consult a lawyer who specialises in property law about what you are entitled to do & if you can ever claim the site as your own.