If someone owes you money the only thing you can do about it (that doesn't involve a couple of big guys with baseball bats) is to threaten legal action and then, if you don't get payment, take such action.
Step 1: Send a formal written demand for payment to your former employer. The demand must be dated and addressed to the relevant person. If the employer is a sole trader or partnership, address the demand to the person (or persons) owning the company. If the employer is a limited company, address the demand to 'the company secretary' (using that title, rather than any individual's name).
As well as clearly showing who the demand is addressed to your demand must also show your own name and address, together with a statement of the reason that you're making the demand and the total sum claimed. Further, it should clearly state that if payment is not received within a specified period of time (I suggest 14 days) you will commence legal proceedings.
You must keep a copy of your letter (as a court may need to see it). The original should be sent by recorded delivery and you should retain your proof of posting.
If you don't get your money, move to . . .
Step 2: Commence legal proceedings. The easiest way is to use HM Courts' 'Money Claim Online' service:
https://www.moneyclaim.gov.uk/csmco2/index.jsp
Alternative procedures (together with general information about claims) can be found here:
http://www.hmcourts-s...bout/claims/index.htm
Chris