hi first is it recognising the hot water demand when u turn on the tap. to check this have the heating off then turn on taps and the boiler should start making noises the try to fire if better it will fire. if it does not reach this process then i would say its the diaphragm or the microswitch. if it does fire and the heating warming then i would say the valve but u changed this then its a faulty pcb. if you want to cut the guessing call british gas they charge a £150 fee incs all parts and labour for a one no fix no fee charge to non members doesnt matter how expensive the fault
