Subletting with lease option contract for three years and now landlord wants property back

Other

This is a slightly bespoke arrangement, but if anyone could advise I would be grateful. A neighbour in a flat adjacent to a flat which I own and lease out asked if I would let his property out as he was leaving the country, he wanted a lump sum also. So we agreed on a three and later a five year period when we would let out his property and pay his mortgage. We brought him to our solicitor and days later we signed a lease option agreement which allowed us to pay a monthly amount and later purchase the property if we decided to do so. The flat needed work which we carried out and we found a tenant.

Now he is sending us abusive emails saying he wants the flat back and is going to change the locks. We have housing benefit tenants and an AST in our names. Their rent is paid directly to our bank account by our local council.

My question is, what measures would the owner of the property have to go to in order to seize this property and enforce our local council to pay the rent to his account?

Any help would be greatly appreciated.

Kind regards

Ben