[KenAdam]

A bit more investigation and I think I may have the explanation.
After typing the previous reply, it occurred to me to wonder why it had written "epub".
I changed the behaviour to have a default output of "mobi" and the next download job was followed by an email job and the file was transferred as expected..
It would seem that installing the update changed the default output format for some reason (and that the wrong default output format means that it will not get converted and emailed).
Not sure if this means there was a probem with the update (or with how I did it!)
Hopefully the scheduled jobs will now run correctly again tomorrow morning
(Sorry - your reply did not show till I had sent this. Still puzzled as to why the format changed, I didn't know about that setting till I went looking to fix it)