Hi Sam!
Right, having spoken to someone in the know (the man behind the bible that is 'Agricultural Vehicles on the Road') there are no restrictions on driving a Fastrac having passed your tractor test at 16, as long as it's in agricultural use and not licensed as a haulage vehicle (i.e. filled up with white diesel instead of red).
But, you may have a bit of an issue with the insurance company - they prefer you to be 17 when driving a Fastrac. So just make sure your boss has informed them and everything should be ok!
Good luck! My first corn cart tractor was a Fastrac 3185 around Windsor. It was my first harvest and i was terrified! In fact, I got done for speeding in Home Park by the Royal Police!
Hope this helps,
Emily, FW Machinery.