I have a mynute 24m which starts and stops?

My boiler is on separate timer and even when the timer is off the boiler fires up and then stops again constantly every 30 seconds or so. The pressure is at 1.5 bar and works with the timer fine but still keeps starting and stopping when it should be off. I hear water movement in the pipes so I believe the pumps are on all the time also. How do I stop the start/stop of this boiler without turning it off at the mains