Sorry folks, no updates as I've been on holiday.
One server now seems to be working OK. We'll find out on Friday whether it manages to remember that it's due to create a new base image.
The other server (the one that was giving me all the trouble with pushing out the remote agent) continues to give me trouble.
I had already tried the article Chris linked to above with no success. It now looks like the backup job/history data has become corrupt so it's a 05:00 start tomorrow morning to follow the instructions at
http://seer.entsupport.symantec.com/docs/319439.htm
Maybe deleting everything and recreating the job (again) will fix it!
Thanks for the tip about the pre-release fix Chris - I'll have a think about contacting Tech Support after I see what happens tomorrow morning.