Damp structural issues in leasehold flat?

Hello, can anyone assist me with a damp issue relating to a flat I bought on leasehold last December? I realised there was a problem which I didn’t pick up on inspection, as the areas of the problem were obstructed.

I didn’t have a survey done as I was in the grieving process and felt I had to be on the move ASAP, I know there is no excuse I just wasn’t thinking right at that time.  I obviously did not bring this problem with me, and my solicitor was very unhelpful. He thought the property was freehold, my flat was built in the 1920s and there is a history of these damp structural issues in my street, at least 6 properties in the last 5 years have had remedial work, and also one person going through this procedure for the third time in the same house.

My adjoining neighbour had the structural issue repaired 5 years ago, but my freeholder said there is no interconnection between my flat and his although they were both built on the same base, can anyone assist me or point me in the right direction please? I was served with a section 20 notice last month, and I dispute his response to my concerns.

Any advice or assistance would be extremely grateful

Thank you,

Gordon