Have had this happen to me before, the plumber probably didn't notice that the header tank is what creates the pressure. Basically Cold water is dumped in the header tank and the "head"  is your working pressure. When is comes out of the header tank it acts as the water main to the Hot water cylinder and the pressure it creates with the "head" onto your Hot water cylinder is the pressure you will get in the shower. confusing??

To keep it simple... If he removed the header tank then he should of run the water feed directly to the HWC (not to header tank) to make up for the loss in pressure. He would then use a pressure reducing valve set to the old pressure and open vent to the appropriate height (or use a vacuum valve). The pressure would then be the same as before.

Maybe he did all this correctly and only has to increase the pressure on the valve?

Hope this helps

Plumber