Honeywell - cm61ng- Keeps going on and off all night

Instructions on how to turn off your optimum start:-

Turn off ‘Optimum Start’
Step 1 Put the thermostat in the OFF position.

Step 2 Press 3 buttons together (at the same time) which are the INFORMATION BUTTON (i) and PROGRAM BUTTONS 3 & 4.
This will show a code of ICL 1 – now let go of the buttons.

Step 3 Press PROGRAM BUTTON 2.
This will now show a code of 30t 1

Step 4 Press the + BUTTON (on the left of the display), tap through until you reach 15 OP 1

Step 5 With the TEMPERATURE DOWN BUTTON; change the 01 to a FLASHING 0. Confirm the FLASHING 0 with the INFORMATION BUTTON ( i ). The 0 will now be static.

Step 6 Put the Thermostat back in the ‘AUTO’ position.
‘OPTIMUM START’ is now switched off.

Mine does the same and displays a little symbol "OPT". I found the instruction manual online and it says

"Optimum Start means that the CM61NG will switch on your heating system so that your desired comfort temperature is reached at the programmed time, thus avoiding unnecessary boiler
firing"

So it may be that you've set your times for when you want the boiler to start heating up the house, not in fact when you want the house to be at that temperature.

Instructions
content.honeywell.com/UK/homes/Technical/CM60NG/CM61%20user.pdf

I'm trying setting the times a bit later and will report back if it doesn't fix my problems.

This is a programmable room thermostat, with timed on and off temperatures. E.g on at 08:00 to give temperature of 22oC, now when you set it to go off, you also need to put in an off temperature e.g 10:00 at 10oC. If this is too high, and i think they are preset to 15oC, it will come one if it drops below this temperature, regardless of what your times are.