Apologies for bumping this thread, but just wanted to say that the solution of uninstalling everything, installing Cross 1.4 bundle and then 1.5 solved my problem. Just needed that slightly bigger buffer size and now all is good. Thanks for your help.
So I got round to recording my first attempts at mixing (no, you can't hear it, way too embarrassed!

) but ran into a few problems.
My sound is coming via my mixer into my PCs line-in port. This is just a question of not having the space for more speakers in my spare room and as the Creative 5.1 system I have is pretty good, I don't need another amp and speakers - saves me money anyway!
I find that if I use Audacity to record my session, after 10 minutes Cross starts cutting in and out. If I view the CPU usage at this point it's not really any higher than if I turn the recording off, but when I do performance comes back. Seems recording to the hard-drive and writing to the recording file causes problems on my old PC.
My solution at the moment is to run another output from my mixer to my work laptop and record on that. But just wondered if anyone had anymore advice? Again, I realise my PC is old, but I'm surprised it can't handle all of this. I've experimented with different buffer sizes and it doesn't seem to make any difference - after 10 minutes of recording, bang, all performance in Cross just disappears.