Combi Centre DX - Mango "boiler keep firing" Questions

...little red button hasnt popped out because this will prvent the boiler firing up- refer to the manufacturers instructions for location of the o/heat stat boiler keeps saying fault on pump/low ...

firing the boiler when using hot water tap.I'm a complete novice so please go easy.Thanks for any help.Alan. could be your pump locking out,or your gas valve.? does it spark,does it light then go out.