Broken Shorthold Tenancy Agreement and Letting Agent Repairs? (UK)?

I moved into a property in 2009 with my partner. After two weeks a leak apeared in the bathroom. The landlord would do nothing after 8 months the ceiling fell in. The problems we had were blowing electrics (all electrics were on one fuse) raw sewage under floors, blocked drains/sinks, no windows could be opened, mould and damp everywhere, the landlord let themselves in numerous times including when I was in the shower! All without notice. On 6th May my toilet ceiling started leaking and finally collapsed 2 days later. Everytime we asked the landlord to do something nothing happens! It was the last straw so we left and moved while witholding the previous months rent and now the letting agent is threatening with court action. Whats what?