Rebecca, you need to give the full circumstances here if seekign advice her is what you are looking for. For example you state above:-
"They promised to hold off solicitors action as they said I had the right to complain, so I did and their response to my complaint is the latest solicitors letter saying remove the treehouse by 6th nov or we will use the possession order inclosed in this letter!"
Are you saying that they already have a (suspended) possession order? If so what is that for? Is it arrears? If so have you complied with the terms?
The reason for the above questions is I find it incredulous that a landlord would seek possession on the grounds of the non-removal of a tree house alone.
That said the advice to see a solicitor is very good advice that you should take