Secure Meters (UK) Ltd - SRT323 - HA display wrong

I have one of the above Z-Wave wall thermostats and it just doesn’t work properly. The main issues:

  • I use HA to change the target temperature to e.g. 19 degrees BUT HA believes it’s 21 degrees, shown in HA. So the Thermostat is now set to 19, whilst HA believes it is 21.
  • It only pings HA every 15 minutes. I would like this to be more often

I’ve been looking through this forum and found a few issues with a similar device (SRT 321) and the quirk suggested there doesn’t help. Any guidance would be appreciated.