I suspect that if I need to get a special 46 to fit the 30 that it won't fit on the 48, so I've attempted to cancel the order. If it turns up anyway I'll try and fit it to see what happens.
I managed to find a full 46/30 crankset for a not-crazy price (£80) imported from Germany so I've ordered that. It gives me the chance to go up to the 175mm cranks which should make hills a tiny bit easier, and gives me a few new combinations to work with.
Thanks all!