Very Slow Flow from Franke Triflow Hot Water Mixer Tap

[Sophie'smum]

We have a Franke Triflow mixer. It was installed in 2008 The HW has never been particularly powerful, We have a traditional gas boiler with HW cylinder & CW tank in loft. We replaced the old 30gal CW tank for a 50 g ready for the new shower. The new tank is a longer coffin shape than the old round one but the base is at exactly the same height. Since the tank change the HW flow to the kitchen mixer is really slow. Just a coincidence? It is not clear from the paperwork that we kept if this mixes in the body but we think it does. It also has a built in pure tap with a filter under the sink but we don't think this has any bearing at all.

The cold pressure has always had a tendency to override the hot. Could this be a combo of scaling up on the hot side & too much pressure on the cold? We are in a very hard water area.

Would appreciate ideas, thank you.