Another ex-landlord here with no sympathy OP. You took on a profit making endeavour and did not dispense your basic responsibilities in ensuring the property was safe or that a proper tenancy agreement was in place. This is why proper landlord registration, tenancy deposit schemes, safety checks and more is a Good Thing. In Scotland, missing these things can land you with really significant fines as well.
I am assuming now that all your safety is up to scratch, the house meets appropriate standards throughout, and that your full paperwork is in order to evidence that? If not, that has to be step 1 before you worry about anything else.
Secondly, you will have to speak to the tenant and seek them moving out on good terms.
The tenant will often get good advice from many housing charities to use the full extent of their rights under the law. And in my experience, councils or charities are not averse to advising people how long they can stay even when the procedure and paperwork is followed (months, until court issues eviction) this is because we are in a housing crises - both a shortage of property and an issue with too many dodgy landlords and overcharging.
The tenant really does have you over a barrel, and indeed if the precedent in law above is correct, you may now have a sitting tenant.
Is your solicitor an expert in tenancy law and practice? I would make sure they are and ask questions such as:
- can you force a new, up to date contract with all the appropriate clauses in, with the existing tenant?
- Would selling the house enable a new landlord to do the same?
- What other leverage is there with the tenant? There can be some sh*tty routes here like putting up rent to uncomfortable levels.
- What creative options are there - including offering to pay the tenant to leave?
I will say it again - basic things like a legal tenancy and doing safety checks is not an oversight, it is unsafe and unsound and places you in the 'dodgy landlord' category firmly. So no sympathy, but I would also understand there are practical steps you can take to explore.