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
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?