If you are through a broker, contact him. If you are not, contact RSA immediately, certainly BEFORE renewal date. If your premium last year was €480 and you were on a 50% NCB, which I think (note think) is a full NCB, then your GROSS premium was €960.
Again, if you have a 2yr stepback, your NCB this year is 30%, indicating a premium of €675.
That is BEFORE any GENERAL increase in premium.
However, if your position is as you state, then you are not being charged the correct premium and you need to contact broker or insurer.
If you deal direct, I'd suggest you write by registered post, to whoever in RSA deals with complaints. You should be able to to get name from Policy documents.