Quooker installation - warm cold water?

So our plumber ended up installing our Quooker tap (wasn’t ready for the Quooker engineer). At the mains incoming where utility is, cold water comes out ice cold (and excellent flow rate). At the Quooker tap it comes out often tepid (and no where near as good a flow). I suspect the flow reduction might be down to a restrictor or something but the tepid water has me worried. A cursory glance and I can see the cold inlet feed for the tap and the inlet to the Quooker tank are branched off a Quooker specific ‘splitter’ (don’t know correct term). 
 

is it possible that some of the hot water from tank could leak back into this splitter and be pulled out when cold tap is running? Or does anyone have any suggestions for tepid water?  Running for a good few minutes doesn’t really make a difference.