When it kicks in, try this site:
http://www.saynoto0870.com/search.php
for the geographic number or an unlisted 0800 number. Also very useful for other companies that try and scam you with 0845 and 0870 numbers. I use it for my mobile co - vodafone, my bank, credit card, loads really.
It doesn't always find a number but there is the option to add one if you have one that's not in their database. And it's quite legit too - got it from the finance guy on bbc radio 2!
Oops! just noticed Jedimaster has listed this site a couple of posts up from mine! Well worth it!