Problem with worcester bosch combi 28siII?

A leak was found in heat exchanger of our boiler, replaced exchanger along with a gas valve, overheat stat, electode and control board. Whilst engineer here all worked well, several days later warning lights starts flashing once per second, locks out so we turn it off and leave it about 10 mins before resetting it. Happens on a regular basis, pressure it okay, e-on engineers do not seem to know what to do. Any ideas