Hot water doesn't run long on Worcester 280 combi?

The heating is working fine - the burner fires up and stays on, but when I run hot water - it goes from hot to lukewarm - the burner clicks on and off every few seconds - my water pressure was just over 2 bar I've reduced it to approx 1.5 but it has not made any difference.

I don't know how old the boiler is but I'm guessing at least 5 but that's probably over-optimistic