How can I sort my thermostat. Code: 20059643?

Override button on WHAT? The boiler? The Thermostat?

I believe the Vokera boiler has a 7 year warranty. You may like to consider that and maybe check before poking around.

Boiler manual: http://www.vokera.co.uk/wp-content/uploads/2015/07/Vision-Combi-25-30C-User-Installation-and-Servicing-manual2.pdf

Thermostat manual:

http://www.vokera.co.uk/wp-content/uploads/2015/11/Setting-the-20059643-RF-Stat.pdf

From the thermostat manual:

TO OVERRIDE CLOCK TO PUT HEATING ON

Press and hold the selector switch for 2 seconds and the icon
will appear and the boiler will operate for heating until it reaches the next off setting
.
To switch off from this, simply press the selector switch again for 2 seconds and the icon will disappear and the boiler will be off until the next on setting

Are you pressing for at least 2 seconds?

laksfjl
January 2018