how to fix worcester heatslave 12/14 clock problem?

when there was a power cut the bioler always kept it's time/day settings, now when there is a power cut it resets itself to mon 12.05 and the boiler does not work until you have reset it. Is there a battery backup for the timer etc and if so how do I get to it to change it. It is an oil combi boiler model worcester heatslave 12/14.