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It is a good idea to avoid the problem of unpaid bills by seeking costs on account at the outset and billing every month on the dot. The client care letter should explain this, and include termination of retainer if any bill is not promptly paid.
Your final letter to the client should always enclose a cheque. Then you have a friend rather than a complaint or litigation. It requires efficiency and purpose, but it certainly works.
If a client should ever dispute a bill ask him/her what they think would be fair. Agree a reduction provided that the amount due is paid within a week.