The motorised valve has V3 micro switches inside it that can stick causing the fault you have, some times as a temporary fix if you latch the bleeding leaver over the heating will work again, however when summer arrives you will find the radiators get warm when the DHW is heated, so only a temp fix.
With the two port valve it acts as a relay, but with the three port valve it is more complex than that, this is how it is wired inside the valve
but the valve has been a standard unit in central heating for many years very common in the 70's, the advantage was the boiler could cool by heating the DHW even when there was no power to the boiler, today boilers have a cool down period built in, but years ago they just switched off, and this valve as default is DHW so it lets the boiler cool as water can still get through the pump when it is stopped.
Today we tend to use two port valves and by-pass valves, which we call S Plan but with the old boiler the Y Plan you have was very popular.
The guy who came to repair the system it seems was not a heating and ventilating engineer, anyone trained to over level 3, i.e. an engineer would have known that was a motorised valve, not a relay, I will admit as an electrical engineer the first time I came across the Y Plan I found it hard, as those resistors and diode inside the unit gave me reading I had not expected. But it was not really my job, although it was a micro switch failed, normally either the motor or whole valve is changed, depends if you can get the motor head on it's own.
But do try latching the bleed lever, that did work with daughters system.
Oh I see that is what the video shows, with S plan it will not cause boiler to run, but with your Y plan it does, or at least it did with daughters, but when summer comes, each time DHW runs it gets the radiators slightly warm, so only a temp fix.