boilerstate replacement for freestanding vulcan?

got new boilerstate which has been updated from the original one, the old onne had two prongs for the wires, brown and a blue to the nutral and another brown to the other pronge. the new one has three prongs 2 brown one blue, i ve connected the two browns and the blue and for some unknown reason the boiler stat wont work, the boiler fires up but you carnt turn it off unless you turn it off at the mains. after 2o min the boiler starts banging and you have to switch off help please