It turned out to be the clutch switch.
I think the reason it wouldn't bump start is that the clutch switch doesn't operate on the starter motor circuit per se - it operates on the engine management. It is not a simple switch, but rather a magnet/circuit board combination that when you depress the clutch bring the magnet and board together, thus doing something that tells the engine management that you can start the car.
The part itself isn't expensive. According to Keith at BHM, getting the pedals and associated bits out to get to it is at least a couple of hours.
So there you go!
Are you sure... i.e have you changed it and it is Ok now???
I had exactly the same problem last year and it turned out to be something even simpler and a recurrign S1 fault.... underated starter relay fuse. Mine went intermmittent and was really hard to spot as blown or not. If your starter fuse is 20A then change it for a 30A and save yourself quite a few pounds.
