Good reminder and solution Wojtek my friend !
I have three Hobbycraft '109s to build at some point, and I vaguely remembered reading something about the tail planes being too narrow in chord.
The solution of altering the slot seems to work quite well, and is acceptable enough visually.
Incidentally, the slot was for the tail plane incident adjuster (the control being one of the two 'wheels' on the port side of the cockpit), which altered the AoA of the tail planes, by the degrees painted around the slot.