I must have been typing my reply as you posted Telstar. I wasn't intending to reconfirm what you'd already stated.
As a rough guide, if I connect via my mobile, read and reply to two or three emails, then read a good few internet pages whilst I'm at it I generally shift about 1/3 of a megabyte. So it costs around a £ to do so. As I say it's too expensive for heavy use, but the facilty to do it maybe twice a week or so (on those rare occasions when turning the laptop on doesn't result in it accidentally finding a wireless connection) is handy to have and worth the £8 or so it adds to my bill a month.
Must try the dial up option to see if that works soon though as call time is cheap by comparison.