Pipelife thermostat ignores instructions

Is it an underfloor system?

Guessing here, but do you have a set-back temperature set near your average temperature? Set-back should be much lower, around 16C, so perhaps it's falling into set-back too often.

Does it turn off the heating once it reaches the set-point?

If it's a smart controller then it will likely use a PID control system to manage the heatloss. PID systems will heat the house and overshoot slightly, then allow for a correction, heat again with less overshoot and then switch on-and-off around the midpoint to maintain a steady temperature. It does all of this by calculating a Rate of Change (RoC) (the number of minutes the thermostat has
calculated it takes to raise your building temperature 1°C) and using that to manage the overshoots.

All of this is done with the input from an external temperature sensor so that the system is operating in a closed loop.

If you're manually managing the heating then you're interfering with the PID control, and the corrections can become excessive and incorrectas the RoC value could be incorrect. The best bet would be to do a recalibrate cycle and then either let it do it's work, or disable the smart control option completely and opt for full manual control.

You might need to check with Pipelife, but there's usually a "un-smart" heating option available in the menu (maybe called "manual mode"), but at the end of the day the systems should ideally be left to its own devices.

https://www.pipelife.ie/content/dam/pipelife/ireland/marketing/downloads/brochures/Heatmiser%20Slimline%20Thermostat%20Manual.pdf

Hope it helps.