How much wider is a 7sp Megarange FW
 

than a 5sp Cadet? Or - will I be able to get away with redishing my wheel?

I don't know the exact difference in width, but 5sp freewheels generally used 120mm O.L.D. hubs and 7sp generally used 126mm O.L.D. hubs. So I would venture a guess that a 7sp freewheel is probably about 5mm wider than a 5sp.