some light 'bang' noice from the boiler as soon as it fires up, pls help?

Hi, I have a Apollo Boiler (approx. 23 yrs old), just recently switched on (was switched off the last 3 months as Summer) but as soon as it fires up, it's making sizziling noise and then some light 'bang' noice, then the boiler stopped. I asked my friends and they think maybe the pump next to cylinder not working, but have checked it is running. Friends came over and helped drained off the tank and cleaned it with Cleanser (they thought maybe scale/sludge blocked the pipes) but still not working. Any advice would be greatly appreciated as it is getting really cold at night. Many thanks.

Sk
September 2008