I wouldn't issue yet, what you should do is write to them with a letter of claim stating your intentions to issue proceedings if you don't receive payments in full within 7 days.
If you hear nothing on the 7th day then call them up, on the 8th day issue proceedings. Claim forms x 3 with particulars of claim x 3. Ask the Court to issue, hand in a cheque for the issue fee (£30 odd for a £500 claim I think).
Hand in the claim form with a covering letter:-
We enclose Claim form and particulars of Claim x 3, I would be obliged if the court could return a sealed copy to the Claimant and serve on the Defendant.
Then the Defendant will have 14 days to acknowledge after the court issue it, and 28 days to file a Defence. They will have to forward a copy of the Defence to the claimant ( the OP in this case ), if they don't, you will and should write in and ask for judgment in the matter.
Which you will get as they failed to response, and then you write to the defendant with a copy of the judgment and ask to be paid within 14 days. If they don't then you can instruct bailiffs to remove goods....etc....etc