Dear Jim
Many thanks for your advice.
However, I do not wish to get involved with the 'Office Insider' program and prefer to wait until the version is 'officially' released.
I have persevered with this and now think I have found the reason. I still think it is a Microsoft issue and something they should look at.
The issue, I believe, is One Drive - or, more generally speaking, the Cloud.
If I try to directly edit a macro workbook on my One Drive, I get the issue. If I download it to my local machine and edit it, the problem goes away.
So, whether it's an issue of Excel communicating correctly with a cloud-based file, or whether it's an issue of the cloud causing some upset within Excel I don't know but I can reproduce the issue by directly editing any macro workbook on my One Drive. All edits to local macro workbooks do not have the issue.
Thanks all, for your suggestions and help. Over to Microsoft...
Geoff