New double check valve and stopcock fitted - water flow issues?

[andreas]

We had our stopcock replaced as it was seized up. At the same time the plumber added a double check valve as they said it was required. Our incoming main is 25mm plastic but drops straight to 15mm at the stopcock. The previous stopcock was a 22 mm stopcock with reducer to 15mm, the new stopcock and valve are 15mm. Since it was changed, if someone turns on a tap whilst someone else is in the shower both flow and temperature change in the shower which didn't happen before (combi boiler). Would it be the double check valve causing a significant reduction in flow? Would it be best to replace them with 22mm stopcock and double check valve then reduce down to the 15mm be more like it was before or would that not achieve anything?

[JohnMo]

Really you should go 25mm to 22mm stop cock, 22mm double check, then get distributed via 22mm as far as possible.

[andreas]

24 minutes ago, JohnMo said:

Really you should go 25mm to 22mm stop cock, 22mm double check, then get distributed via 22mm as far as possible.

Thank you