no hot water VAILLANT turbomax vuw 242e, 282e?

i have a vaillant boiler and have had no hot water properly for a very long time. my rads work fine. I had a diverter valve changed about 5 yrs ago.
i have had 2 engineers out and also one diagnose over phone. 1 guy came out said diverter valve and so did the guy on phone (the one on phone changed diverter valve last time though) the other guy said its a blow out switch. looking on here a switch is about a tenna, and to to change diverter valve ive been quoted £280. How do i know who is being honest with me. ive got 2 small kids who are having cold washes and i dont know who i can trust and i need this sorted asap