I have a gearbox in bits, maybe you could identify the offending items as postage might kill us.
Hi Darrell, thanks for responding so quickly!
I've opened up the gearbox and found .. no damage, and all the parts intact!
Basically I think the problem is the input shaft on top. It seems to be able to move - the bearing is sliding out about half a centimetre towards the front of the car, and when this happens this input shaft meshes with other gears and jams things up. (When in second gear, at least)
Also, I think the syncro tabs on the 2nd gear, lower pinion shaft were not held in properly. The spring clip seemed to have slipped off and when I removed the hub, they all fell out.
I put it all back and tried the gears out by pushing the selectors with my hands, and it seems to be fine - no jamming up any more but I need to prevent that shaft from sliding.
I'm not sure if I need any new parts at the moment, but cheers for posting. I'll post this up on the mech problems forum and see what people say.
Thanks!

Alastair