Which is why this website might save you a tidy sum
http://www.saynoto0870.co.uk/
Basically, all these 0870 numbers have standard pilot numbers which will cost the same as a normal national call or occasionally they may even have an alternative 0800 free number that they don't advertise.
Just type in the 0870 number or the company you want to ring into the search, and it'll come up with the cheaper alternative for you
Very handy site to bookmark, thought I'd pass it on!