I was all geared uop to wail that I'd tried that...
Then further investigation revealed that while I'd been blithely switching the realtime scanner off on the server and the clients, the admin program for the network-antivirus had been blithely switching it all back on again as soon as my back was turned. Computers!
Fixed it it in any case and as a bonus it's also solved the problem from a while back when I was having difficulty access the published exe's themselves at any sort of sensible speed.
Cheers.