Well after a rather long Christmas break, I have returned to the Panzer III.
I made some modifications to the rear deck, removing one of the ventilation structures.
I used the depression on the underside of the errant ventilator and drilled a series of holes, joined them using a scalpel and then filed it all square.
I then glued a cover piece on the inside.
And then filled the cut out with some more plastic card.
I used milliput to smooth out the surface (a Crooked Dice Corporal shares the workspace).
It has now been undercoated with PSC German Panzer spray paint.