Simon, look at Rukka Rain Warrior waterproofs / over-suits; 2 piece jacket and trousers are quite easy to get on in a hurry.
Not horrendously expensive but it will make you sweaty if you wear it in warm weather,
But over my Hein-Gericke All Seasons Jacket it works really well.
(Although my PC800 Pacific Coast keeps most of the rain off once you're moving at any speed.)
Interestingly, the trousers seem to be more generously cut than the jacket so try it on if at all possible.