Yes, it's all on S plan.
Would the megaflo have come with it's own valve?
When my megaflo was installed, they simply reused the existing honeywell valve that was already there.
Same with the heating valve, the existing one was reused.
I don't think the DHW valve is faulty (it seems to travel back and forth the same as the heating valve right next to it, and it also 'feels' the same when you operate the lever manually.)
I've checked the wiring, all appears fine. It's only three wires, and I've tested that voltage is applied across the valve terminals depending on heating or DHW demand.
There isn't even an immersion wired up to my megaflo, so that can't be it either.
I'm going to run the heating without any hot water at all for a day or two. So once I've used up the hot water in the cylinder, there should be no hot water available at all.
If after a day's worth of showering and washing etc, there's still hot water from the cylinder, then that will confirm that somehow the heating circuit is also heating the cylinder.