Right I thought, that can't be too difficult, whipped out my C550 and checked the T-Mobile settings posted on this site. I managed to navigate my way around the phone, eventually finding entry points for much of the info posted.
However, it still won't connect to GRPS. I couldn't find anywhere to enter the 'dial-up' number and I reckon this can be the only thing left to do.
Anyone know where it should go?