Here's an update on my "mystery noise" - it was the freewheel. Just recently got back to France, changed the freewheel and problem solved.
With hindsight I could have pinpointed the culprit earlier - taking the freewheel off by using the removal tool in a vice it was clear that there was a lot of play in the freewheel. I could probably have detected it with the freewheel still attached to the wheel if I'd known what to look for.
The new one is 11-30 which is a much better range for my daily outing, though the rear changer needed quite a bit of adjustment to get it to run smoothly and it still skips on the 30 tooth cog occasionally - needs a bit more adjustment.
Thanks to everyone for all the advice - it's great to be back on my bike!
Will post the first part of my review soon.
All the best,
Phil