rotten problem - non rotten advice please!!!

Maintenance

8 years ago I spent £20k doing up a 2 bed terrace.  We found bad dry rot under the front room floor. My builder replaced rotten floor joists and floor boards and treated lots of timber and upgraded ventilation.  But we did not get to the bottom of the problem.  I think our mistake was failing to treat the brickwork.  Slowly the plaster on walls is crumbling low down.  

Current tenant of 5 years is middling.  What I really want to do is move her on and address the building problems with an empty house.  I served s21 which expires soon.  Meanwhile she has complained to Manchester Council about disrepair in order to scupper my eviction.  I have tried a couple of carrots to get her out ;