Not 100% sure about the cut down flap vanes but I was always under the impression that it was cut away to preclude the 'possibility' of the flaps affecting tailplane effectiveness at slow speed on 'some' jets ie engineering/production tolerances may have come into play.The RAF accepted the resulting (5 knot ?) increase in approach speed.
The 'SMURF' came about (I believe) during the T45 design phase because obviously HS wanted to achieve the slowest approach speed but also with good tailplane effectiveness at low speed.
If you look at pics of later mark Hawks landing - the L/E of the tailplane is usually approx lined up with the smurf so I assume the smurf directs air over the t/p in some way or other.
Whilst working at a certain airfield in surrey
I did actually assist one of our design team to attach a cardboard 'SMURF' to the side of a Hawk fuselage with masking tape as they worked towards the T45 design
.