As can be appreciated - the shape is dictated by not wanting to have two servos ... straightening the hinge line allowed me to use a joiner and single servo.
The real Hunter of course has split stab / elevators - but the elevators are parallel chord and swept back .... being a sub-sonic flyer.
Another has suggested PVA / Paper combo to stiffen the foam ... instead of paper - I would use light dress material ....
At present still considering replies. The problem seems to be really one of when I push her at speed with 4S .... so maybe simplest remedy is to revert to 3S ??
I also wonder if the high mounted tailplane may be 'moving' in total .... it may benefit from having a triangular gusset glued in either side of stab ?