Well, out of interest we went live with FCGI yesterday, and things have been *much* better. Not only are we not getting random crashes, but the response time feels more zippy too. I suspect the problems I was having configuring it were down to my trying to serve from the same location as another Apache instance running Apache2Handler. Perhaps it should have been obvious that the two were going to conflict! In the end I opted for some downtime, and after much fiddling with settings (and longer downtime than I had originally promised!) I got it working. No out-of-hours calls were received last night, which was rather nice.

Thanks Chris again for your assistance. Meanwhile using prefork on Windows would be interesting, but now that things appear to be working, I think I will leave them alone. I'd much prefer to be using Linux, but it remains to be seen whether I will be allowed to specify this for our next web server ;)