Thanks for your help-apologies for the delay in getting back to you - but we are only just getting up in Perth, Australia.
I tend to agree with you that this is a one off, only time will tell, but it does seem odd for a file to suddenly lose its xml attachments.

Just for your information, I am not saving the file as a template (.dotx or .dotm) and that is the whole point: rather I am saving it as a .docm with vba and xml contained within the file. This enables a user, anywhere in the world, to download a copy from SharePoint and save it (as a .docm) and retain the vba and xml.
If I used a template, then the downloaded file would need to be able to see the template in order to access the vba and xml. This, of course, it can't do if the template is on SharePoint.
Anyway, thanks once more for your kind help.