Andrew Gabriel wrote:

I'm confused as to the difference between GU10 bulbs

Mains - plugs in a bit like a fluorescent lamp starter.

Right, I thought I had seen some LV GU10s but obviously mistaken.

Are the pins then 10mm apart

and some GU5.3 bulbs I have.

LV (12V usually) - two pins 5.3mm apart.

That makes sense but how on earth did they arrive at 5.3mm, the pins
hardly seem precision items and I'm sure 5 or 6mm would have fitted,
maybe I am wrong.

Geoff Lane