same as Dazmond and above,
don't post a letter through, just clean them. that's what we do and don't get any complaints whatsoever. just a query now and then (maybe two in the past 6 months!!!)
also, i don't like this letter as its written from the perspective of the WC... i am a business and need my income etc etc... why should the client be interested in this? the real point is that the results will be exactly the same, and we are so confident of this that we'll come back if you re not happy!