H-N-H bond angle in ammonia boron trifluoride adduct

In the reaction:


$\ce{NH3 + BF3 -> NH3-BF3}$ does the HNH bond angle increase or decrease? 


Surely, the FBF bond angle decreases because boron changes from $\ce{sp^2 -> sp^3}$. But how can we comment upon the NHN bond angle? In my first attempt, I expected it to increase because though N still remains in $\ce{sp^3}$ hybridisation **lp-bp** repulsions are greater than **bp-bp** repulsions. 


But the answer key states that the NHN bond angle will decrease. Please let me know how to determine this. 


I couldn't find the bond angle data on net or in JD Lee.


