If someone owes you money, the only thing you can do (which doesn't involve a couple of guys with baseball bats) is to make a formal demand for payment and, if payment is not forthcoming, commence action through the courts.
Send the following letter, using recorded delivery:
"Fred Bloggs
Re: Non-receipt of mobile phone purchased from you, and your subsequent failure to refund monies paid to you.
TAKE NOTICE that I hereby DEMAND IMMEDIATE REPAYMENT of the sum of �XX.XX
Further TAKE NOTICE that if I do not receive payment of this sum within FOURTEEN DAYS, I will commence COURT ACTION against you.
Yours . . . "
If he doesn't pay up, start here:
http://www.hmcourts-service.gov.uk/infoabout/c laims/index.htm
Chris