Desperate for help

This is my first post so please excuse any errors.
I have an invoice which I submitted on 29th March 2012.
My invoices clearly state 30 day payment terms on the bottom of the invoice along with some wording which I got from this site about failing to pay within 30 days.
This invoice remains ooutstanding.
I am a fitness instructor and this invoice is for classes I have done at the local leisure centre.
I have resubmitted this invoice at least once
I spoke to the operations manager this Tuesday 15th April and he assured me that he would clear payment and get it sorted that same day and that a cheque would be posted.
I contacted the manager again yesterday Thursday 17th April and he said again that payment had been authorised and that it was on it's way.
My question is this
What would you do next ?
How would you word it ?
I am at my wits end this is the last invoice I am waiting for before I can submit my accounts for the last financial year.
Please please any help would be appreciated
Thanks in advance.
Jon

Hi Jon,

Assuming this invoice is still unpaid, we would recommend you use this letter as a final reminder.

Chase this up with a phone call no later than 3 working days after posting.

If they still have not paid it may be time to look at small claims or speak to your accountant about carrying the invoice over into the next financial year.

Thank you so much much appreciated.

So when I add the 8% interest on do I add it just once or do I go on adding every month or week for as long as the invoice remains unpaid ?
Sorry if I sound stupid !

Hi Jon,

No such thing as a stupid question :)

You can recalculate the interest every day if you wish, but once a month is probably sufficient. We have a client that includes the increased figure on monthly statements.

Thank you again.