Poor hot water flow?

Heat Line boiler...central heating working fine, but very low water flow/pressure through all hot water taps.

On initial investigation boiler pressure was up to 3.4 bar, as filling loop had been left open. Bled off down to 1.5/1.7 bar.

Is there a filter on the heat exchanger, that may need cleaning, or could it be a worn/holed diaphragm?