Not sure if this site has been mentioned before, but its brilliant.
You just tell it the expensive number you've been given starting with 0870 0844 etc and it finds you a cheaper one.
And what I've found is that you get a much better service via this alternative number - rang to get a repair on my my Hotpoint tumble drier via an alternative number and, rather than waiting in queue and eventually getting through to a harassed person in a noisy call centre I got straight through to someone who was incredibly helpful and efficient and clearly not in a call centre 
Give it a go.