Shower pressure - cold

We had our low pressure hot water tank replaced about 6 months ago with a mains pressure tank - which has done wonders for the hot water pressure around the house.

But for some reason the cold pressure in the shower (only) has always been rubbish. It’s great at every other outlet in and around the house, even the other taps in the bathroom.

Is there common diagnosis for this (ie. the cold was being restricted to allow for low pressure hot, or the mixer cartridge is damaged as Dr Google suggests?), or is it impossible to known without a plumber investigating further?

TIA for any suggestions