Well - perhaps 'chuck it back' was a bit on the loose side - suffice it to say that I've placed the ball back in their court :)
When you say is FSUIPC running - I have to say yes it is - it is selectable from the Add-ons menu and I was able to register it without a problem. I registered FSX successfully without any issues at all. As I said, when I got my new machine (you may recall we touched on it on the RC5 group) I just installed FSX and the BAV OSP to eliminate any extraneous programs, but still it wouldn't play.
The log file is attached which I believe indicates there's not a problem (I hope).
I appreciate your time, but as you suspect it's something to do with the OSP itself.
Thanks again.
FSUIPC4.zip