Vokera Linea 28 Digital Clock setting?

Hi, looking for some advice please

My mother in law's boiler Vokera Linea 28 (6/7 years old)
keeps losing its settings, she had a new clock fitted 14 months ago. In the last month she called heating engineer round as boiler not working & he re-set clock settings and charged her for the visit. I have since then re-set the clock 3 times to get boiler working again. Does anyone know if the clocks are problematic & are they easy to replace? Any advice would be very helpful. Regards Peter