19C seems pretty cold to me... ?
Do you use the HR92 as the temperature sensor for the zone or do you use a remote wall sensor like a DTS92E or Y87RF ?
If you're setting the set point lower during the day and higher at night it sounds like you are using just the HR92 as sensor and unwittingly trying to compensate for the inaccuracies of measuring room temperature beside a radiator.
In short, the colder it is outside and the hotter the radiator is, the more you have to increase the set point to maintain the same "comfort" level as the HR92 senses a temperature that is higher than true room temperature when the radiator is very hot.
Conversely when it's warmer outside and the radiator doesn't need to run much or at all the temperature reading at the HR92 will drop below the average room temperature because it is close to the floor and usually near an outside wall. So you then find yourself having to turn it down to avoid heating when you don't need heating.
If you use a remote sensor you will find that you don't need to vary the set point during the day or even with the seasons. We have a DTS92E sensor in the living room sitting near the opposite side of the room and schedule the living room to 21.5C through the whole day on the weekend and mornings and evenings during the week.
We don't change this set point for summer or winter either - with a remote sensor there really isn't any need to, as provided that it is properly positioned in the room it is measuring the true room temperature. In winter it doesn't matter if it's day or not, mild or freezing weather, it maintains a comfortable room without adjustment. And in summer when the room will naturally heat up near this temperature it still turns the radiator off even when set to 21.5C and avoids overheating the room.
Prior to installing the remote sensor we were setting the Living room anywhere between 21C in warm weather and 24C in freezing weather trying to maintain the same perceived level of comfort.
Personally I think a remote sensor is worth it for any room where you spend a lot of time as it really improves temperature control dramatically over using the HR92 as sensor and eliminates the influence of direct radiator heat on the measurement.
The way the temperature is rounded and biased for display is the way Evohome has always done it, and the way the devices like radiator controllers and remote wall sensors do it. As those can't have their firmware updated there's nothing that can really be done about it, certainly not in a firmware update to the Evotouch itself, which is the only device in the system which can be updated.Todays test highlighted the weird algorithm when you increase the heat by 0.5C the displayed temperature jumps by 0.5C a few minutes later.
In my test the actual temperature was obviously to high 17.3C to actually request any heat. So why doesn't the display actually show 17.5C so that increasing the temp by 0.5C will actually cause a request for heat. I know that the radiator valves will show a different temperature if this software change is made. This is a beta release so input from users should be considered overall this release is much better no overshoot of temperature.
I don't like it either but I've come to accept that it's just how it works. If I want to get a precise room temperature reading I look at my graphs which use the raw temperature readings.