Main Combi 30 HE - hot water only with heating, not on its own

[secretlife]

Hi there. Please could you advise? I have the above boiler and it is 9 years old. For the last few months you can only get hot water if you have the heating on as well - which has been fine in winter but if we do get hot summer weather, it's not going to be any fun having to have radiators on simply to get warm water for washing. If you turn the switch to hot water only, it doesn't fire up - but it does if you turn the switch to heating.
Is there anything we can try ourselves or do we need to get someone in?

 

[bazdaman]

Faulty diaphragm for sure

 

[adlplumbing]

mostly likely the diaphragm as above

 

[Agile]

Or the switch!

 

[secretlife]

Thanks very much for your replies