Well, it went in to the garage yesterday and they 'stripped and regreased the rear cv joints'. How much stripping went on I don't know - I think they probably took a circlip off one end of the boot and squeezed some cv grease under the lip. So we wait and see - no vibration yet but it's early days. I'll let you know!
What 'Wally' didn't understand was that the vibration was 'intermittent' and only happens '1% of the time' - however, as instructed, I have driven it at 30 and then at 70 (no vibration), I have gone at 50 and then braked hard (no vibration or shaking - the steering wheel has never shaken anyway), and the gearbox is as smooth as the proverbial, whatever you do with it.
So, touch wood, (or wood effect plastic in the Lexus' case) we may have found a cure. Thanks to all those who have given advice.
Regards
John