I can only advise on the Small Claims Court procedure. This would cost you �60 to make the claim if you do it online (
http://www.hmcourts-service.gov.uk/). I believe you can add the fee to the claim plus an amount for interest. You can work out an amount to charge for interest by going to this site
http://www.payontime.co.uk/calculator/statutor y.html
This will give you the statutory interest rate that companies use for late payment of invoices, which I would think would probably be suitable for your case.
A trick for getting payment without actually making a claim would be to get the paper copy of the Court papers, fill them out and send a copy to the company owing you the money. This usually tells them that you are serious and it saves going to Court etc if they pay up. If they dont pay you can still just send the papers off to Court to make the claim.
Good luck.