they say :

Hi Andrew,

The spammers are simply watching the whois for new domains. We don't sell our customers details to 3rd parties but we do automatically set-up a catch all email address which means you will receive the emails they send. the only way to stop this is to set the catch all to bounce all unknown users. the spam will generally stop after a couple of weeks though.
Best Wishes,

Alan Whitehead
123-reg Services
www.123-reg.co.uk