Surprise tenant . Advice needed on unauthorised tenant and legal position.

Access

We bought a property in 2008 with a tenant in situ (sale and rent back) . In 2018 she died . When we tried to access the property we found another "tenant". This person claimed to have always been living there , unknown to us. He then tried to take legal action claiming ownership of the property and false title transfer on the basis that the lady we bought the house from did not inform him she had sold it. We have proved this was not the case as the file proves that the conveyance was correct. However, we have been unable to gain access as the tenant is not answering calls or letters . We have been trying for years to access in order to do standard legal requirements, safety etc. We have contacted both him and his solicitor , who apparently still acts for him . Although we do not know in what capacity. His solicitor has ignored calls and messages since July 30th this year and the situation is just ridiculous. We need access and possession of our property . A solicitor we used advised us that we are all legal in respect of the ownership and that a tenancy technically exists . He does actually pay us £550 per month and has done since we found him there. But any attempt to evict has been met by solicitors advising they will fight it in Court. Whichever way we look this is an expensive situation and one which we did not create. We simply cannot afford to pay a solicitor at £210 plus vat per hour on this. We are also advisedly local agents that the rent is currently low in relation to the correct market rent. So one view is that we increase the rent to market rent and if it is not paid we can seek possession for arrears. We really need some advise.