Hi
Just to let you know, that I believe I have successfully cancelled the advert with Yell.com. The woman I dealt with there is called Maria Bridgen, she's was very apologetic and accommodating.
It might be worth emailing her at Yell.com – I don't have her full details, but she can be reached through Customer.Service.Team@yellgroup.com and see if you have any luck.
I will of course be adding this my blog, in hope that it informs other people to be wary of their sales people.
Best of luck.