Vokera 24e Combi boiler hot water fault?

Good morning,

I hope you can help.

I have the above boiler and for some time now the hot water has taken a long time to come through to the tap.

I have recently had the diaphragm changed but it hasn't improved things. My engineer says he dosn't know what the problem is.

I have another engineer coming out next week, but would like to know what the problem might be so I have an idea of what to expect.

Any help would be very much appreciated.

many thanks

John