It's got all of the components of a C-Plan system, but it's not wired up like that!

[M51]

@ianmcd
Thanks for pointing that out Ian.
Other than it not being a correct placement for the valve as per the C-plan standard, I'm not sure what the real world consequences are of leaving the valve where it is, do you know?
It's actually been there for the last 20 years by the looks of it. Under the old wiring configuration, when the DHW stat was up to temp it used to close the valve anyway and still run the boiler for the central heating.
There is some sort of vent on the DHW flow (you see it on the left of the photo that I posted) not sure if that's supposed to help somehow?

 

[ianmcd]

In the real world would probably be ok, but if you are ever bleeding rads or anything like that, you will have to manually latch it open or no water will enter the system, as per your own post

The zone valve should be fitted in the DHW primary circuit so that, when closed, it does not close off either the cold feed or vent. It is preferred that the valve is fitted in the return to the boiler."