SX spur gear moving away from pinion under acceleration?

[QuackingPlums]

Took my SX out for its first run today (yeah, I know... procrastination is my middle name ) and I ended up stripping the spur.

When I picked up the car and spun up the motor I could see the spur visibly moving away from the pinion! It's fine when I slow down again, so I'm thinking there are more than 3 screws I need to tighten to fix the mesh. I can see two underneath the rear bulkhead with the round heads, and the one behind the idler gears. There's one underneath the motor that I can reach through the slot in the chassis, though I gather that's just the motor clamp. They're all tight, so what's left? I haven't got a manual, so any suggestions before I go download one and find out the obvious for myself?

The car isn't brand new - I bought it off ebay and from what I can gather, it's only been run once or twice. Nowhere near enough to have bent a layshaft or anything... I hope!