For some time there has been a banging sound from under the car.
I finally got the car up on a jack.....this is what I found....

A quick trip to Williams this morning and Tom told me it is surprisingly common, it is usually one side only, but can be both. It seems the exhausts are not symmetrical and can move over time.
A few minutes with an air driven grinding disc followed by a rotary file...

Noticeably quieter.
Laurie and I also had a good look under the car and found one of the rear exhaust hangers, a rubber block with a Jubilee Clip round it, had a broken clip. Quickly replaced.
Otherwise a few scuff marks on the Woolf sump shield, no damage to the inside faces of tyres or wheels, but the rear tyres are down to the wear bars in places. I've been noticing tyre squeal when pulling away "quickly" , and the occasional twitch: time to replace the rears.
Well done Williams.
As ever tea, friendly chat and a spaniel.