First the best news: it's working again!
Thank-you to all for your advice.
I am sure now that I must have done something wrong during installation of the JSGME update....just what, I'll never know.
Shortly after my last post my power unit burnt out, so I've been out of touch while my PC has been in the shop.
I took advantage of the opportunity to upgrade my hard drive from 40 GB to 500 GB!
So last night I reloaded the game from scratch, checking each installed version step by step along the way, and no problems now.
