Hi my name is ***** ***** will help with your question today.
Depending on how the unit was setup there are different ways to override the boiler function. There isnt a specific override button on the unit, however you can configure it so when you adjust the temp the boiler will come on regardless of the timer.
As I am not sure how your unit was setup, my suggestion is you have a read through the manual so you know how it could work, and if you have any issues enabling the features you come back, and I can talk you through the process once we know what you want changed.
The manual can be found here
Have a good day.
If the timer is set to A/B, then page 22 details the user overrides.
Simply increasing the temperature using the up and down keys should result in the boiler starting. Below is taken from page 22:
Temporarily alter current programmed temperature Press Λ or V until required temperature is displayed. Please note that your installer may have restricted both upper and lower temperature settings and the temperature override limits. This override will automatically be cancelled at the beginning of the next programmed event.
Another option is to put the timer into thermostat mode for the period you want to override, this is detailed on page 23, and can be toggled on and off by pressing the up and down arrows together for 5 seconds.