Nearly there!
UST fitted and soldered correctly. New bone pre-shaped bridge saddle bought (£2.50) and sanded to fit (height + length).
It works... BUT the centre strings have greater output.
I 'think' that as I've sanded the underside to the correct height, I've potentially applied more pressure to the outer edges and hence the centre is a whisker deeper/higher. I've also profiled the bottom face/edge to a 'U' shape as I'd seen somewhere that if the bridge tilts even a fraction in the slot (which most will) you get less contact area on the UST; maybe I need to flatten that to get a bigger contact area.
We shall see.