vincevincevince - 10:33 am on Nov 7, 2009 (gmt 0)

You should not have agreed to give an hourly breakdown to justify your fee unless your fee was based upon hours. That was a mistake on your part.

Invoice today, via signed for post, fax and email, for the full outstanding sum with short terms (e.g. due immediately / five working days), with whatever interest penalty terms you can legally apply. Don't give them 30 days unless they already have a record of paying promptly.

When the money has not been paid for two weeks, write to them (signed for post, fax and email) requiring them to contact you immediately and explaining that you may turn the debt over to a collection agency if it remains unpaid.

When they contact you and ask you to do X, Y and Z before you get paid, inform them that the invoice is in respect of work already completed and must be paid in full before you will do anything further, even providing breakdowns, screenshots or technical details.

Odds are, you will not see the money even if you do turn the account over to a collection agency. But it is worth a try! In any case, do not do anything more for this client!