It appears, then, that this person is saying one thing to you (alleging you are dishonest) and another to the agency (alleging you are uncomfortable to work with).  

I think you have made the right decision about leaving as it would be impossible for you to work with such a person.You may wish to consider, however,  taking this further to clear your name.  

This is a game that this person is playing with you, and although I am not a betting woman, I would wager a great deal that she has done this before.  Dealing with her nastiness is not the issue here; clearing your name is the important thing.  

Either way, this person has damaged you quite thoroughly.  On the one hand she has made a certain allegation - wholly unsubstantiated - to your face which will no doubt be repeated to select people in your company, "well ... there WAS talk about such-and-such a thing.  We couldn't prove anything, but she left fairly quickly ..." To the agency she has said you don't get on with your colleagues - which may damage your chances with them.  Again this is unsubstantiated, but still damaging.

London is a much smaller place than many people think, and the legal community, from my experience, is quite tightly-knit and things get around, particularly when one works at a certain level.  

You may, of course, be just looking forward to going and never seeing Ms Back-Stabber again, and chalking this up (or down) to experience.  Whatever you decide to do, Jess, I wish you the best of luck in finding a new and better job.  

Best wishes,

Raindance