Hi Arb,
No, it's not an FN.
The bike is much older than it looks - possibly as old as 1910 or even earlier! I can spy an early open-magnet magneto and the tram-handle for an NSU 2-speed gear on the mainshaft of the motor.
That said the bike has been thoroughly modernised with Brampton Bi-flex fork, disk wheels, sidecar etc.
Let me guess the photo was taken immediately after WW1? There was massive inflation after the war and for a couple of years bikes were hard to afford. Easier to make your own!
Cheers
Leon