although in this case, it seems that on his planned trip, he will be in places with a range of 0c to 30c, so would only want to take one bag.

azza, if you figure you "sleep warm", then you probably have to advantage of perhaps going the route of a bag that is good to, I dunno, lets say 5c and the times on your trip it is down to 0c, sleep with all your warm clothes on (as suggested by others). This way, the sleeping bag could perhaps be smaller and less bulky--and/or not to sweltering in warmer temps---and you will have a toque (winter hat) warm pants, fleece top and all that stuff anyway with you, given what riding conditions you will be in sometimes.

I guess you should try to figure out how often on your trip it will be colder, and then go from your experience with sleeping bags in varying temps in the past. Like others have said, everyone is different, being skinny, I'd go more towards a warmer bag, I've had some cold miserable nights in not-warm-enough sleeping bags, and its no fun.

Perhaps a warmer rated bag that works for you could save a bunch of money too, although I dont have any suggestions of specific bags for you.