Primary pipe work meaning the pipe work after the DHW valve?Sounds like you're losing heat via the cylinder coil, possibly through gravity circulation.
At a guess DHW zone valve permanently open - seized or wrongly wired.
I'd start with feeling the primary pipework for warmth half an hour after switching off rads and hot water.
I’ve felt the pipework after the DHW valve while the heating has been on and it’s cool suggests the valve is working ok?