vokera compact overpressure problem HELP!!?

I have a vokera compact 28 boiler, the safety valve keeps blowing on overpressure,the filling loop has been disconnected (thought the valve was passing)but still the same. The only way I can stop it is to shut the cold water mains inlet valve. Looks like mains water supply is getting into the radiator side, does this mean the heat exchanger has gone?? and is leaking internally?