Hi Marcus
Try
www.reestitmutton.co.uk or
www.mobilechecker.com
I have used them both; I always go for deals with either OSPS or E2save
(both part of OSPS).
Personally, I would not risk porting in a number after taking out the
deal, as my understanding of the t's & c's is that the phone number on
the invoice and all bills submitted for cashback must match.
Regards
Simon Clark
Business Telecoms