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From what you tell me, you certianly have grounds to obtain "possessory title" to the garage. Under the adverse possession laws, a party can make such an application if they have occupied/possessed land which does not belong to them, for 10 years (if the land is registered at the Land Registry) or 12 years (if the land is not registered), where such possession was not with the express consent of the true owner.
You should therefore contact a local Conveyancing Solicitor to make the application which has to be made on Land Registry Form ST1. This Form asks various questions about your period of possession and once completed, it can be sent off to the Land Registry, with their fee of £40. If a Land Registry Surveyor needs to inspect the Garage, there will be an additional fee of £50 to pay.
Once you have been granted possessory title, you in effect become the legal owner of the garage.
I hope this assists you and sets out the legal position.