how do i replace a DHW Diaphragm on a baxi 105e?

When the boiler is set to either central heating or hot water the boiler does not fire up when turning taps on for hot water. (But the central heating works.) No hot water comes through taps, unless central heating is manually turned on at the boiler, (making the boiler fire up). The tap light does not illuminate at any stage. The central heating works, and I have also tried re-setting the bolier the same problem occurs. Any suggestion would be greatly appreciated.