I think a lot of people are confused about the concepts of 'premium' and 'no claims discount'.
If your premium is, say £100, and your No claims discount is 75% then you will pay £25.
If your premium increases to, say £1000 and your NCD remains constant at 75% you will pay £250.
Given that your premium is determined by what is a very sophisticated actuarial process, and in the main, you are unable to protect it, your NCD is just a marketing swizz..
When someone drove into my parked car and admitted that it was their fault the claim went through their insurance and had no effect on my premiums. I'm with Aviva BTW.
I have to laugh at this - are people really so naive and solipsistic that they believe this to be true?
In a single case, it is entirely possible that a no fault claim won't have any effect, but if for example, 3 people rear end you in a 12 month period, then this does raise issues about the safety of your driving. Although none of the incidents are technically your fault, it does suggest that you stop abruptly and are therefore a higher risk. Hence your premium is likely to be higher at next renewal.
Even if you have had your stationery car repeatedly driven into in car parks (the ultimate in 'no fault') it still suggests you are less safe as it can be inferred that you park in a manner that leaves your car vulnerable.
The algorithms used in the actuarial process are incredibly sophisticated and give a fairly good predictor of how likely you are to cost the insurance company money at some point.