Is there a problem with my thermostat or the boiler?

I have an Ariston EuroCombi. I don't know it's age, as we opnly moved in 18 months ago. It's occasionally temperamental but hasn;t cause major problems - little things like needing re-igniting or the pressure sometimes drops a bit. However, these last few days it hasn't been starting up unless I turn my thermostat up. It has been at around 20C mostly, but is now at 30. It may be my imagination but I also feel like the boiler is tripping off more quickly than normal.

I have little or no clue about these things, so any jargon-free answers would be appreciated!