Oh yeah, burner and pump on same connection Would there be any issue wiring it like this. When the CH is on the sw live would backfeed through cyl stat and bac to programmer. And of course the cyl stat won't be able to control temp when CH is on, only when HW only is on. Or is it better just to hang on the second valve, purely for the electrical operation, not mechanically doing anything, and wire it as standard S plan system?
One you said oil boiler, does it have an over run to cool? Mine does not, so for me no option have to use thermal syphon for DHW so boiler will not over heat. As said you show no thermostat, maybe you don't need one? however if using TRV's then no need for motorised valve. But the original central heating at parents house was an as well, not instead of other heating, it warmed up house in the morning before the fires were lit, and it was as you show with no thermostat just time clock. What you show will work, no question about that, the question is will it do what you want?
You also need to consider how much it will cost to run. Without a thermostat on the heating, the boiler will keep its set temperature until the time clock switches off, regardless of the temperature of the house. I had this setup with my original gas boiler (now on oil). Adding a room thermostat halved my gas bill, as it prevented the boiler cycling when the house was up to temperature.
The householder only has the heating set for an hour in the morning and evening, he's a farmer and won't be in house much during the day. Not depending on room stat to control temp.
I don't know if the boiler has an overrun to cool, would need to ask plumber. There was a valve for rads in the hotpress, when the householder could manually shut it off in summer for HW only, now there will be a motorised valve fitted there. My only concern is when the CH is on the water in the cylinder will be heating until the burner shuts off....but i suppose thats the way it originally was! Only change really is the ability to have HW only if required.