hw pressure probs

I have just installed a worcester junior 28i into my mates dads place replacing an old glowworm fully pumped 'Y' plan system. the problem is the hw pressure is poor it comes out well when the tap is first opened but gradually reduces to a very poor flow rate with 10 -15secs.(this is the same on all the hot taps). The shower works fine im guessing because there is a bit more back pressure. Now how can i rectify this waqs thinking of maybe running the cold water in for the cw storage tank instead of mains what do ya think?

There's a flow restrictor disc in with the mains filter and turbine flow sensor, on the left inside. It's common to take the restrictor out, because these boilers scale up really quickly if the HW gets to the magic 60whatever it is (63?). 10 minute job, but have a skinny 1/2" fibre washer ready for the water pipe at the inlet end. The tap connector size ones are too wide.