Following earlier posts under the topic 'Reverse Engagment', I think I've got things sorted.
As mentioned there was a lot of play in the clutch release arm, both in the push rod and the buttons carrying the release bearing. I ordered a used replacement from Rimmers, but this had almost as much play as the old one so was returned. Fortunately I have a lathe so I turned up some bushes to take up the wear. After a quick gearbox overhaul (see circlip and synchro posts) I got it all back together yesterday. I also replaced the slave cylinder. Intital test in the garage seem fine. Clutch well up pedal and no grating into reverse or first. Can't try it on road just yet - need to shuffle cars round to get it out. Now the interesting thing is that it looks as though there was a clutch problem well before I rebuilt the car and I didn't notice. The clamp bolt slot in the old slave cylinder has been filed out and is nearly 1 1/2" long. I guess someone did this in the hope of getting more movement by pushing the cylinder further in.