I'm probably tempting fate, but I think we've solved this issue; we've been running over a week without any problems, which beats any of our previous experiments.

We have quite a high legacy of extended store procedures, OLE automation and distributed queries in our application, all of which use RAM outside of the main memory pool. This can be configured using the -g start up flag - under 2008 it defaults to 256MB, but under SQL 2000 it was 384MB. Having restarted the database with -g384 added to the options all now seems to be OK.

Cheers

Jon