if you're a NAMED driver and do not hold the policy in your own name, you have no NCB. If you are with, say, GIO, they have their own *INTERNAL* NCB schemes.
They told me that they give me an increase in my GIO NCB at half the normal rate of increase (e.g. 1yr no claims normally means 10% increase, but I get 5%). If I get my own car and get a GIO policy under my own name, they will honour my internal GIO NCB, so I don't have to start from scratch.
But it doesn't mean jack to any other insurance company. It's their discretion whether or not to honour it.
Hope this helps.