what causes the low pressure light to come on, on a worcester 350?

it has a low water pressure switch inside, which when tripped will illuminate this light and will kill power to everything else (programmer included).

Either

a) there is low water pressure, check the pressure gauge and look for around 1.5bar

or
b) the switch has failed or is dirty (happens commonly after you repressurise the boiler) and needs to be cleaned or replaced.

Bunslinger
May 2009