Many thanks for the fast reply.
It seems I was pursuing ever more complex solutions when the problem was elsewhere - what you have sent works perfectly to generate the output.
The problem appears to be with my phone (Samsung S7E, Android 8) in that it will not actually send the attachments in native email, Gmail, Dropbox or Google Drive. WhatsApp is the only one of the available apps tried so far on my phone which does actually export.
Possibly some sort of security issue?