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