>>>If I start a case which county court would the proceedings be held in Portsmouth or Leeds
Neither. It would probably be dealt with by in Northampton. That, of course, assumes that you make your claim online, which is both quicker and cheaper:
https://www.moneyclaim.gov.uk/web/mcol/welcome
Remember that no court is likely to issue a Judgement in your favour unless you can show that you've made a formal demand and that your demand has been ignored.
So the first thing to do is to send a formal letter to the company at their registered address (which might not be the same as their head office address). That letter must show your name and address and those of the company you're claiming against. It must also be dated. The letter needs to clearly state that it is a demand for payment, specifying both the amount demanded and the reason for the demand. It should further indicate that if payment is not forthcoming within a specified period of time (I suggest 14 days) you will commence legal action. The letter should be sent by recorded delivery, using Royal Mail's online tracking service to confirm receipt.