Many thanks Tom, I think you've answered my question.
But here is a follow up question... This all came about because I tried to fix something which probably wasn't broken in the first place. Doh!

In my efforts to tweek FSX I went into nHancer. I suddenly realised I had an FSX profile which had FSX.exe as the executable AND an FSX duplicate profile which also had FSX.exe as the executable. nHancer flagged this by turning the profiles red. I saw no problems with my FSX FPS or image quality, but this still struck me as something wrong which needed to be put right... am I correct???

I then went and deleted the FSX.exe from my non-modified nHancer profile and changed it to manual activation. Now I see a very noticable drop in my FPS.

Any ideas? I'm now gonna go back and set it as it was and see if my FPS is restored...
Many thanks
Andy