In a 26 October 2006 post I noted that an AVG anti-virus program update had fixed some Email problems. It just happened again! Spooky...
The latest AVG update (7.5.503) appears to have not only restored the Email scanner without disabling PGP but also cured the problem I was having with not being able to send Emails via my ISP's server, though weirdly the Google Gmail server was fine. I've had my Email accounts happily Emailing each other PGP encrypted messages anyway.