Why does my gas back boiler fire up every 20-25 mins when thermostats are low and 'holiday' mode is set?

You may have the setting on the taps set to a specific tempature and if the tempature of the hot water drops below this then it will fire up to get the tempature back up to what you have set it at. Just turn the tempature down for the taps and it should stop, it means that the hot water will take a few more seconds to come out of the tap but it will save you more money in the long run.