How much does it cost to call the tax people in UK?

I've been in a que now for half an hour waiting for the inland revenue to pick up. I know they don't want to give me back my tax, but I'm a bit worried the phone bill I'm racking up is going to be insane. And it never ever ever just takes one phone call. Two years ago they made me cry because I sent them all the forms and they still wouldn't send out the check and every time I rang I waited for ages and then they said 'oh, we thought we'd sent it' . Is an 0845 number free on saturdays? Or is that just after 6 on weekdays? How much am I rougly paying per minute here? How long do you reckon they'll take to pick up, given that I've already waited half an hour?