Boiler problem further data on earlier question ?

Thank you for your helpful responses. I have made further observations and these are as follows:

I have two hot water tanks each serving parts of the property. Each tank has a motorised valve in line. The central heating system also has a separate motorised valve in line. Both of the hot water valves seem to be in the off position – the leaver on each is in the right hand position. If I move the leaver to the left I meet resistance at the half way point. I have moved on of the valve leavers fully to the left and locked it in this position. The pipe either side of the valve is cold. With the time control set to “24 hours” and the central heating set to “off” the boiler still refuses to stay on – it comes on for say 20 – 30 seconds and then cuts out. If I set the central heating to “24 hours” the boiler burner ignites and the radiators get hot.

I cannot visually follow the pipes from the hot water tanks back to the boiler [as there are numerous pipes all of which are lagged] therefore I am unsure whether there are any additional valves/pumps in line.

If I am correct that both motorised valves are off it would suggest that there is an electrical fault somewhere in the system. However, I cannot understand why with one of the valves locked in the “open” position this does not result in the boiler burner coming on. I also have no idea where the electrical switch to the valves is.

Does anyone have any further thoughts?