Need more information to make educated suggestion rather then take shot in the dark
1. Circulation taking place between cylinder and boiler while boiler power is off
Question here is, with MCB in off position, the hoiler is depowered, but are the controls also depowered
2. Have been involved in installations where boiler is dead but pump still runs as controls are powered from another circuit.
3. I would be looking for a pump and take it from there. Instead of boiler heating the water in the cylinder, looks like cylinder is trying to heat the boiler as pump is running all the time.
4. Is the boiler above ( higher) than the cylinder? 10m distance suggests mechanical circulation rather than gravity. More so if bulk of the pipe run is horizontal.
5. Service valves under the boiler- disturb this at your peril as most regulars here will tell you. Not unknown for these to start leaking/ weeping when these are operated. Why makers cannot make valves that are fit for the purpose, One make comes to mind where the spindle in the valve snaps When the valve is disturbed.
Can be done but be aware if the pump is causing the circulation as isolation will stress the pump.