Thanks for the reply. I had already tried that (admittedly not with showerhead removed).
With the pump running I did get a slightly higher flow into the en-suite but nowhere near what it would normally be (although didn't
know whether this was because most of the pumped water was going to the...
I have a Jetforce 55 (MK6) shower pump in my airing cupboard sited next to the hot water cylinder.
The main bathroom (adjacent) has a bath and shower, both of which utilise this pump (i.e. if I'm running a bath or have the shower on you can hear the pump running).
Our en-suite (further...