The little LED Voltage display actually reads a little low compared to what a proper multimetre gives. 12.45 on the metre = 12.3 on the Mini display. 12.4v on the metre reads 12.1 on the display. So I'm a bit happier now that the batter although is technically a little low seems to be charging sufficiently. So.. when in use the display is 13.5, but what is more likely 13.7v.
I will at some point put additional earth points on the engine and frame just to reduce resistance in the circuitry. Not sure if it will do anything, but I have a huge box of wires to which I can experiment with this pursuit.
I may or may not put LED bulbs in the system. I'm undecided yet on that.
As for the 9ah battery, that is standard for a etz250, however my other etz250 worked perfectly fine for years on a 5.5ah battery with no indications of voltage loss (at least by indication of the dis-charge light).