I have been offered 2 different funded PhD's. With everything else being equal, the scholarship for one of them is substantially more than the other. What would be the best thing to do to ensure that the scholarship amount advertised is the actual amount that I'd receive if I accepted? I'm not asking because I'm greedy but because I think I would struggle to live off the less generous scholarship.

Although horror stories of people ending up with less than they thought exist, they are actually quite rare. To approach it tactfully phone or email the supervisor and ask him or her to confirm the scholarship details, as it is a higher one than you've been offered elsewhere and wanted to check that you'd understood it correctly.

Thanks for the advice. I did subtly enquire before, but I was told that the amount of the scholarship would be confirmed upon my having accepted the offer. Actually, the other PhD offer was likewise advertised as being at a rate of standard research council funding + a top-up. But when I went to the interview it emerged that this probably wasn't going to be the case. Hence the reason for my suspicion.

how much is the stipend offered by association for international cancer research? im too shy to ask my supervisor!

Your supervisor was a PhD student once too and will be perfectly happy to tell you how much the stipend is. Ask them - if you discover they are a monster or that you are pathologically unable to talk to them, it's best you find out now.