Changing my alloy wheels, anyone know the answer to this?

You need to keep the rolling radius the same so size of wheel not as important as size of tyre to prevent rubbing on arches and to keep speedo accurate. Also, if you are putting wider tyres on you again need to decrease the profile. You also need to check the pcd of wheel studs against new wheels for fit and also the shape on hub to shape of rear of wheels. If in doubt do not buy and go to a specialist-the tyres are the only thing touching the road and if you get it wrong you won't stay on the road for long!