I have never had problems after sending this letter. Payment usualy arrives pretty swiftly. 
Dear Sir / Madame
Despite numerous attempts at recovery your debt for our services remains outstanding,
This will be my final request at recovery personally.
If this debt is not cleared within 7 days I am left with no choice but to take further action.
My normal course of action is to first refer the debt to a recovery agency, this will add a cost of £75 to the outstanding amount.
Should they be unsuccessful and fail to settle the arrears then I am left with no choice other than to refer it to Her Majesty’s Court Service.
Please note: Interest will be charged from the date the debt became due as set out in the Late Payment of Commercial Debt Act 1998.
Additionally we also report any outstanding arrears with credit reference agencies 28 days after the date of this letter, which may affect your ability to obtain credit in the future.
Current outstanding debt £xx.
If you wish to discuss this matter personally please do not hesitate to contact me,
Yours sincerely.
Neil Gornall
I'm not a expert but I do not think this is legal, you have no credit agreement. I currently have a customer who owes me for 2 cleans at £15 each. and I looked into this the other night. You have to have a license to be able to charge interest etc.
http://www.oft.gov.uk/OFTwork/credit-licensing/;jsessionid=2EBBF41E056BDD3678BDF225EC076251From what I understand it would be illegal to charge interest without a license from the OFT
So what can I do about it.. not a lot I just have to suffer the loss and like everyone else says find a new customer.
I have posted a payment reminder letter but it's nothing like yours.
Dear Customer,
May I remind you that the amounts shown above are overdue for payment.
I would ask that full payment reaches me by the 30th September 2011 to ensure my continued service to you.
Please note that no further reminders will be issued.
I have provided the details of my bank account below, if however you are unable to pay via transfer then please contact me to make arrangements for a cash/cheque collection.
If you are unable to pay or have any queries regarding your account, please contact Lewis Hill
Our line of business relies on trust, you could try and take them to court but then it's your word against theirs and they could just say something like "we never asked you to clean our windows", "we cancel our agreement" and many other things.