1st non-paying client. Can anyone help?

Hi

Together with my partner we have been trading for just over a year and everything has been great. 

We then get a client who hasn't done any bookkeeping, payroll, returns at all since December 2008.  We agree to take on the work as the client has all of the paperwork.

We complete the 1st year and send the client an invoice, we move onto the second and complete that.  We send the client another invoice and receive a chq by return.  The chq bounces.  During this time, we are working on year 3. 

The client sent another rubber cheque.

We have now completed all 3 years, bringing year 3 slap bang up to date, payroll and VAT.  The client says they are waiting on a customer paying them and definately will pay. 

With the state of the paperwork, trod on, ripped, coffee stained faded etc, we spent over 60 hours on this and now can't seem to get paid.  The client is adament they are good for the money, just not yet.

The client is also asking for their paperwork back.  My partner wants to hold onto the paperwork but I disagree with her.

Does anyone know if we can legally hold onto the clients paperwork until they clear their account.

Many thanks

Jon