You should certainly get 2 years NCB if you don't claim. The proof of NCB often comes within the wording of the renewal quotation so it is important to keep this particularly if you are renewing elsewhere. If the insurer believes that money is owed on the policy, they may not send you proof of NCB.
If you make a claim, your next premium will be higher. It's usually best not to make a claim if the incident is one that can be sorted out without involving insurance companies. The effect of a claim on NCB often depends on the policy, the claim and whether NCB is "protected". With some policies, you can claim for a windscreen repair without it effecting NCB. With other policies, the NCB is reduced by 2 years for each claim, with others, NCB is not reduced if it is clear that another driver was 100% at fault.
The rules are not set in stone but it is usually the case that, if you make a claim, you will end up worse off after 3 or 4 years than if you didn't make a claim irrespective of NCB rules or protection.