No Hot Water

Hi,

Everything has been working fine with our 6 yr Gulfstream boiler but over the last few days, we are experiencing a problem with the hot water.

It seems that each morning there isn't any hot water available? However, throughout the day everything seems ok. Is this because we have the heating (all works fine) on which in turn is heating the water?

We have recently replaced the heat exchange plate but other than that it's all still original parts. I have noticed that the boiler doesn't seem to fire when the shower has been running (which used to happen). I'm not sure if this is related? Could it also be that the water pump needs replacing?

I just need a fix before I replace the whole system next year!!!

Thanks