Domestic Hot Water has a low flow rate on Vaillant AquaPLUS?

We've had our AquaPLUS combi boiler with 15l Power Store since 2005. However today the Domestic Hot Water (DHW) has a low flow rate, but temperature is still as expected. Cold water to all parts of the house are as normal. When tap is turned on, the initial discharge of water is normal (but only say half a second) then returns to a low (say 20%) flow rate.

Display on boiler states that pump and ignition is working, and fans/pumps can be heard on the power store etc.

Have checked for air traps in the radiators (especially in the bathroom towel radiator as highest in the house), but no luck.

Any ideas would be appreciated.

Many thanks