Old_Jim obviously likes to do things the old-fashioned way. Most people would now claim online:
https://www.moneyclaim.gov.uk/web/mcol/welcome
(It's usually quicker and cheaper).
You should note however that no court is likely to make a judgement in your favour unless you have first sent a formal written demand to the debtor, and that demand has then been ignored.
Your demand must show:
(a) your name and address ;
(b) the name and address of the debtor ;
(c) the date of the demand ;
(d) a clear statement of the amount demanded, together with the reason for the demand ;
(e) an indication that, if payment is not received within a defined period of time (I suggest 21 days) legal action will be commenced against the debtor.
The letter should preferably be sent by Recorded Delivery, so that you will be able to prove to a court when it was sent (and that it was received).
See here for further information about the claims process (including, in Section 6, the steps for enforcing a Court Order if the debtor ignores it):
https://www.gov.uk/make-court-claim-for-money/overview