no hot water or heating even though boiler fires up?

worcester 28i
boiler fires for CH & DHW
but DHW & RADS cold
when you open a DHW tap the boiler fires, right?
OK if you leave DHW tap running for 5 minutes does boiler fire continuously for the same length of time?
when you turn CH on for 5 minutes does boiler fire continuously for the same length of time?
if you answer YES to both questions then i am struggling to understand where all the heat generated inside the boiler can dissipate to?
if you answer NO to both questions would need more info ie does boiler make banging clattering noises? any fault codes/lights displayed?

manwithnoname
May 2012