Worcester - programmer/timer fault (on 350 Combi boiler)?

I have a Worcester 350 Combi boiler with a (Worcester? - it's their name on the little booklet) single channel programmer.
We've had problems with the programmer 'losing' its settings, certainly the clock has been wrong. It might be due to a power cut. The Worcester website says "electronic programmers have battery back up" but there's no mention of it in either the basic booklet for the programmer or in the Installation & Service Instructions.

QUESTION (finally)
Is there a battery?
How do I find it? Do I need to isolate mains supply?
Any other tips or things to watch out for?