hiya chris, i used moneyclaim couple of months ago. It says about interest:
Q. Do I need to tick the box about reserving the right to claim interest?
A. If you wish to claim interest in you claim you will need to tick this box and then copy and paste the pop up box that appears into the particulars. You must ensure you complete the brackets in this paragraph correctly to enable you to claim interest.
Q. How do I work out the interest I am entitled to?
A. If you are claiming interest under section 69 of the county courts act 1984 you must multiply the amount you are claiming by 8% and then divide this by 365. This gives you the daily rate of interest. You must then work out how many days have passed since the date the invoice was due until the date the claim is issued. Not only must this be stated in the particulars of claim, but this figure must also be added to the amount you are claiming.
When making the claim, i didn't add interest and i put who i was claiming against as the claimant and the defendant, but she paid up within day of receiving the letter from the courts.
Because i'd filled in the form wrong, the claim was invalid anyway. But she paid + the £35 charge to register the claim.