Finally had time to look at the exhaust power valve today.
I removed the left hand side power valve cover. I pushed the power valve bearings all the way in and then, using my digital caliper, measured the gap to the retaining washer, or rather the face that the retaining washer sits on. It was 1.8mm.
Luckily, I found a suitable washer 1.6mm thick. I placed the washer between the inner bearing and the part that pivots and turns the power valve. Thus leaving 0.2mm clearance to avoid jamming the system.
It worked great, the rattle has gone. Just been for an 60 minute ride and all is fine.
I think this is self explanatory, but if anyone needs to see a photo then just ask.
All the best, Dave.