Hi Paul, and welcome to the club. We have similar cars down to the year! I am not an engineer, but I had a similar problem in a Rover some years ago. The cause was a damaged tyre and the vibration came and went. It could be worth checking the tyres out, particularly the tront tyres. Also check that there is no play in the steering. The other cause of vibration could be something loose, possible the exhaust. Regards, Peter.