Hi
I've had the problem a few times but, touch wood, not whilst window cleaning.
As per the post above, go via the Small Claims Court.
The process is very simple, very informal and based, surprisingly enough, not on complex 'he said this and done this and all that TV type thing', it is based on common sense.
One claim I successfully made was against a local Southern water supplier. The relative size of the company concerned as opposed to small you is of no concern to a small claims court regardless of how well they put their case. Believe me.
Just go to any local Court building and ask how to make a claim using the Small Claims Court. They are always helpfull even if you are in the wrong type of court building.
One other point that is often missed by a lot of people, the CAB will not help you with a claim, they will only tell you how to go about making a claim with the Small Claims Court. The CAB do not take sides but instead provide valuable 'how to' advise.