Detective work?
Credit companies will use any sneaky tactic to get a hold of a debtor ...from another credit account that has been passed to them or a related company, contacting your family/neighbours/previous addresses, credit reference agencies. If you owe money ...and more often than not even if you don't. They find you, and harass you until either you a. pay up or b. make them prove your owe the money/the debt is yours.
You could try registering your number at these peeps:
http://www.ukdatait.co.uk/cgi-bin/sgAddNumber.cgi