Hi guys, I think I've found out what the issue was.. and I have to admit its pretty embarassing stuff.
Somewhere along the line I changed the option
Simple Track (no car shadow) from "off" to "scenery"
Now, don't ask me why (I'd have thought enabling any form of simple shadows - whether it be for just "scenery" or "all" would yield an increase in performance) but selecting any option other than "off" on my PC massively kills the framerate - suddenly I'm back to 80fps on the Blackwood stretch again.
Well.. this discovery obviously makes me happy, (although I can't believe the time I've wasted messing with settings, drivers, opening my PC etc) - but I just wanted to thank everyone who tried to help me with my problem - nice work boys. (^_^)