Saunier Duval F28 E2 problem?

Hi

I have a Saunier Duval Isomax F28 E2 boiler which contains a water tank that holds approx 45 litres of hot water.

The boiler is 7 years old.

For a couple of months now, a problem has started when I am using the hot water, as soon as this tank is depleted, no more water is being heated up on demand to continue a supply. If the tap, shower etc, is shut of for about 10 mins then another tank full of hot water is available for use, again until this runs out.

There is no problem with the central heating, that works fine, also no loss of presure or any error message being reported on the control panel.