Finally got round to getting this sorted today!

Work perfect, massive thanks to Mattaustiin for the solution.

FYI - how I got it to work...
- Turn wireless off on Nexus 7 (N7)
- Pair devices via Bluetooth
- Under 'settings' on BlueVPN (N7) select access point and enter the APN found in the N9 under settings> Internet Connection > Edit Networks > *your mobile internet network* (in my case its - general.t-mobile.uk)
- In BlueVPN select the paired N9 device (it will start to show its trying to connect)
- When the big 'power' style button comes up on BlueVPN you are connected!

Mountains of WIN!