Update on this for the benefit of others. Increased the pipe size attached to the 32mm incoming main from 22mm to 28mm. And increased the megaflo output to 28mm as well as the balanced cold, for approximately 2-3 metres - whatever was accessible in the garage.
This has made all the difference. A single shower feels much better, but more importantly, even with all 3 showers running, they are all extremely good. I wouldnt say there is no drop off in performance, as there is. But it's not as dramatic and because performance has increased anyway, the drop is now on par (or better) than what I was getting with just one shower previously.
A few notes which are important:
1. I have a 22mm/ 3/4" bsp water softener sitting in between the 28mm pipework at the start. To put paid to the myth that there is no benefit to increasing pipe size downstream.
2. My gut feeling is that the 28mm pressure reducing valve has had a big impact on performance. Looking at the haetre sadia 22mm one, the orifice is tiny - how they claim to get 70 l/m through that, I don't know.
Hope this helps someone else.