Did you check the pump as there are usually 3 speeds on them.If it is on 3 turn it down to 2 and see what happens.If the thermostat was at its highest setting then it could have acted similarly to a boiling kettle and thats why it has been pumping over.Hopefully with the boiler stat on about 1/2 way and if need be the pump speed turned down a notch,you should be able to stop it pumping over.
Tony S
January 2009