The advice I have always been given is that we can hold our 'working papers' if we have not been paid for them because they are our property but we cannot hold the client books and records as they belong to the client.
In my experience, the outgoing accountants will hold onto everything until payment is received.
To enable the incoming accountant start right I would normally expect to supply the following :-
Closing trial balance of last accounts
Copy of last accounts
Copy of last tax return
Closing TWDV's for capital allowances