Can they produce the weigh bridge slip?
My trailer(Custom trailers) weight is 150kg on the makers plate, granted it is for a Ski Vee 500, but it is a normal double bar, break neck trailer, galvenized.
As willem mentioned his weighs 220 kg so max would be 250 kg for yours if you have the same boats.
What I can tell you is that your trailer cannot weigh 550 kg! unless it has been built with solid steel!
If they cannot prove inconclusively the weight of the trailer, then let them weigh it again in your presense.