If your boiler is condensing, The Wickes Ace HE 102, it does not have a diverter valve. It may be the DHW flow sensor or thermistor. Have you tried to see if the central heating side is working? If it has the same fault when in heating demand, it will be something else, possibly the flame sensor, air switch, fan or??. There’s quite a difference between a plumber and a heating engineer. Some can do both, some can not. Be mindful of this.