Fiona,
I worked in the Insurance Ind for a few years. If you take the NCB protection it means your stepback (the amount expressed as a percentage) that you are pernalised by should you crash is limited i.e.
Full NCB & no protection = (Crash) next premium looses its usual 50% reduction for 5 yrs NCB. Full NCB & protection = (Crash) next premium looses only 20% reduction, still availing of 30% reduction (remaining balance of 50% for 5 yrs NCB)
FBD tend to be good value for money depending on where you are living in the country, they are usually at the competitive end of the market when you are looking at your renewal premium. I would ask them how much the "stepbak" penaly is with the full NCB protection and how much without?
In the event of a crash you will not be able to transfere your policy for 3 -5 years or if your do you will end up paying a very high premium. Ins companies need to be able to recoup some of their costs should you crash and they work in a cartel-esque manner.
Hope this is of some help.
Regards,
Sambo