No idea if this is still the case or not, and please do NOT take it as "gospel" ...
My brother was in this situation some 30 odd years ago, so things have probably changed, but in his case he spoke to the CAB (or whoever it was then) and was told quite simply ...
The terms of his lease still applied (he had a 2 year lease with the option to give 3 months notice, the landlord had no such option) even though a management firm took over the property and despite their "forceful" letters they had no rights or ability to evict him. He was also advised to tell them that any further attempts would result in a formal accusation of "harassment". This he did and it all went very quiet, he remained in the property for a further year or so before moving.
I obviously don't know the terms of your lease .but I would suggest a quick visit to the CAB armed with a copy of it would get you all the information you need

The management/trustees what ever should already have copies of your paperwork .. it is NOT up to you to supply them.