Having just fitted a new bathroom, I now find that the hot water flow rate
is about half as it used to be, what could have caused that?

System is a standard boiler with hot water storage cylinder. Pipe work for
bath is 22mm, washbasin is 15mm.
Pipe work is virtually unchanged from previous setup, save for the inclusion
of isolating valves.
Taps are connected with flexible hoses and I have checked those for eventual
kinks, their fine.
Had to drain the system to make the connection and the drop in pressure has
happened since then.

Any suggestions of what could be the course and solution would be much
appreciated.

Henk