If that's 6ltrs cold inlet flow, that seems extremely low, what sort of pressure/flow at the hot taps have you got
What pressure is the incoming cold main to the house, somethings amiss here
Just thinking also, that coil could have been your problem before you had it pressurised and no one noticed it, you've now increased pressure and can now monitor the loss, where unforeseen before
Possibly the coil in the cylinder as suggested, it would either back up cold feed to Header tank or mix in with hot side, so when you turn a hot tap on you could be emptying the heating side,,,,I think
More than likely nothing to do with the Plumber, you need to get someone to check your boiler install.
Back flow pressure on mains tripping flow switch on boiler
Good Plumber will sort it