• Sorted!

    I'm surprised I hadn't already tried that as its just the sort of cover-your-front,back-and-both-sides belt&braces that I normally go in for!

    Reset back to zero and reconfigure took 7 minutes to complete. I have absolutely no idea what it was up to ... but after setting back to 1 and reconfigur'ing a test sp_send_dbmail worked straight away.

    Off to check now that it is indeed really fixed, rather than just pretending to be fixed!

    Thanks, you've saved me days of messing around looking for answers.