You need to amend the agreement for future.
It needs to say, “if the rent If the Rent remains unpaid for longer than 14 days then the Landlord can peaceably re-enter the property and exclude the Tenant and the Tenancy Agreement will immediately terminate”.
If that clause was in, then you could simply let yourself into the property and lock the tenant out. You still have to give him his belongings back and you would have to serve him a Torts Interference with Goods Act notice, giving him 30 days to get his belongings and telling him where they were and when he could collect them. That notice is better coming from a solicitor.
If you think he has disappeared off the face of the earth, (he could be imprisoned or dead) then you need to do everything you can to find him and that would include giving the notices I mentioned earlier (if you Google the phrase it will give you plenty of reading) and put a notice in the local paper asking for anyone with his whereabouts to tell you and asking him to come forward.
Putting the same requests on social media is also a good idea because then, if you do dispose of all the goods at auction to defray the outstanding rent, and he decides that whatever was in the garages is worth millions of pounds, you have proof that you try to get hold of him. Get a 3rd party with you to take photographs and do an inventory of exactly what is in the garages.
Can I clarify anything else for you?
I am happy to answer any specific points arising from this.
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