Good stuff Geo, and what's this 'sleep' thing? Haven't had more than a couple of hours at a stretch in the last four or five days!
The part you have circled looks to be from the Tamiya kit, supplied as a separate part for ease of moulding. It's the top of the frame, and the cross brace, immediately behind the rear glazing, and would be visible on all high-backed Spitfires. The part mainly visible is the cross brace, and this can be replicated by a piece of plastic strip, drilled, or even painted to represent the lightening holes.
The harnesses shown appear to be the later version of the Sutton harness, and were fitted to some late production MkIXs and MkXVIs, as well as late production Tempests, and Meteors etc. There's a possibility they might have been fitted to your subject, and to be honest, in this scale, they would pass as acceptable.
On the Spit, the 'tail' strap coming off the 'Y' shoulder straps, passed through a rectangular slot at the base of the head armour, just above the top of the seat, and reached almost to that cross brace. Twin 'Bowden' cables were attached to a metal plate at the end of the tail strap, and passed over the cross brace, and were anchored deep in the upper, rear fuselage, via an inertia lock and pulley system, the release for which was down by the pilot's right thigh, virtually invisible in this scale.
The voltage regulator on this Mk, if fitted, resembled two cans of beans, mounted one on top of the other, transversely across the rear of the head armour, but, as far as I know without checking, these were on the earlier production MkIX, and replaced by a different type of VR, within the fuselage, on later models. Have a quick look at photos of the Squadron's aircraft, or even others of the period, which should indicate whether or not it was mounted.
Hope this helps.