hi we just had an engineer and he said because the water was spitting out of the autoair vent he said we need a new water to water heat exchanger,an expansion vessel, a pressure gauge and a filling loop cos when he tried to drain it, it leeked from the expansion vessel as it's a combi boiler worcester 28 cdi can u help and let me know what to do as we have been in this place a year and the boiler is only 4yrs old and was regularly serviced before what is your opinion please thanks,Mrs B