I have just signed with npower, and I was sent my document confirming this. The contact number is 0845 601 1102. I called the sales number (0800) to complain about this and threatening to cancel the contract if they did not provide a geographical number. After a brief "consultation" the girl who took the call gave me the customer sevices number (also 0800) and told me that I could use that for any calls I need to make to them. I have not yet tested this out, but when I have done so I will post it.
She would not be drawn into a discussion about "local/premium rate" stuff, just kept reminding me that I had the freephone number, so I suspect she was told what to do, and what not to do. (Note that it is possible that this may be localised ie specific to my geographical location, although I can't be sure). For information the number is 0800 553 322.

Thanks Heinz - I am in Cheshire, so perhaps not!