I've done loads of these in the past and to be honest I wouldn't touch one again with a barge pole unless I was absolutely desperate. There have been a few, and I mean a few, OK ones but most of the nightmare stories that stand out in my twenty plus years of ind/com cleaning have been builders cleans. Here you'll find the spawn of Satan manifest in the form of the BUILDING SITE MANAGER. In my experience, window cleaners are considered scum by these Delboy love children. The really bad one's will try absolutely anything to get out of paying you, and the bigger the building firm the worse the BUILDING SITE MANAGER seems to be.
I can reel off brown trouser stories all night so to keep it short, I'll give a sort of 'worst case scenario' checklist.
Paperwork:
Make sure you have all relevant paperwork, CIS, health & safety etc.
Common scenario:
They'll ask for such and such document/s as you're nearly finished. If you're missing something, they can throw you off site with out payment.
Accusations of theft:
Have someone check your van/s every time you leave site and get it signed for, times, dates everything.
Accusations of work not done:
Get every section you've done checked and signed for, ideally by the basta.. I mean the BUILDING SITE MANAGER. You'll always get someone in sanding or whatever after you've finished somewhere.
Accusations of damage:
A particular favourite of the BUILDING SITE MANAGER. You can keep this in check while doing "Accusations of work not done" above.
Sorry to put a downer on things but if you get a wrong un' he'll do you right up the er... wrong un'
Good luck.
CIS -
http://www.businesslink.gov.uk/bdotg/action/layer?topicId=1074014514