Suddenly my shower is hot and cold?

In the last few days my shower has started going up and down in temperature. I first noticed it struggled to get hotter than luke warm, now it goes up and down. I have a specific shower dial between my taps

Boiler was new when I moved in 4 years ago (EVO HE), and my neighbour told me that apparently these boilers are not good ones (?). I have never touched it and the thermostat always remains low. My heating is only used minimally during the colder days so not much at all throughout the year.

The heating switches on as I tested it last night (although didn't really notice if my flat got warmer).

Any advice? Hoping I don't have a big plumbing bill coming!