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?

JIM, January 2009

There is battery back up on these timers BUT.. the battery only lasts ten years or so, and eventually it quits. It is soldered onto the timer and cannot be replaced.

New timer m'afraid.

Bunslinger, January 2009

not familiar with the 350 in particular but the timer unit itself will usually have a battery (similiar to car key fob battery) either inside or mounted on the rear. All you need to do is locate and replace. Batteries available from larger Boots stores or motor manufacturers

tojo, January 2009