After 2 years if annoying rattle, I eventually got my local dealer to confirm the fault. Car is just over 3 years old, so they wouldn't fix it under warranty (18 months only for rattles, etc it seems.....).
The dealer master tech was great to be fair, had a good chat, talked about this thread, what had been found before, suggested fix and he then fitted the suggested O-Ring to the gear selector rod and exchanged the metal washer for a nylon washer and also applied silicone grease. I now have a lovely non-rattling gear stick above 4.5k revs. Cost £115, but worth it.