Manage to get the bottom of it after alot of faffing around...
My inner/boot tail light LED wasn't working because it's live feed is taken from the live feed on the outer tail light. The outer tail LED driver board had previous water ingress causing that LED to partially illuminate but not enough to notice it was at fault (at first). The LED driver board was also not outputting enough juice to supply the inner LED.
I took the lights out and because I couldn't find a wiring diagram I buzzed out and took the voltages across the contacts, that gave me enough to know that outer LED feeds inner LED so I buzzed out those link wires (marked in blue in the picture)....all good apart from lack of volts on the passenger side.
Anyway, replaced the entire outer cluster on the passenger side which now kicks out the correct voltage to the inner and happy days!
If anyone is interested I've attached my scribblings for my variant of A3. It shows the cable colours, connector pins and continuity between ground and live with lamps inserted. (with cluster and bulb housing removed from car)...hope it helps someone out.
Thanks for all the suggestions
