Thanks johny,
really impressed with the details and usefulness of everyone's replies. I'll do what you suggest above. I've ordered bits for the linkage mechanism which should improve things a lot, but it would be good to know I've checked the selectors before going through the pain of replacing the assembly. A few years ago I had to replace the clutch twice in a year (because the back plate on the engine was warped the first time), so I have some experience of refitting the gear box etc. At least I know it goes in eventually!
I'll report back when its all back together but it might take me some time to get the rest of the car working again after its long rest!
Thanks again
Colin