Mark,
Many thanks for your response. I'm not able to do that at present. I'm not in France, where my mower is, and I won't be until early April. I'll aim to pick this up again when I get back.
Current status is that I put a further new belt on and the clutch now works OK. The previous belt was wrong size despite having the correct part number. I have redone the valve timing. It now starts OK. As the weather was so bad I was unable to get out and fully test it.
Regards
David