New Triton T80 shower: Increased flow trips pressure switch: so only hot showers!

Just installed a Triton T80 10.5Kw Easi-fit, to replace an old Aqualisa. The old shower didn't have a 'cool' opttion, so in summer it was always too hot, even on the low rate option. The Triton has a cold option at least. However, it's beginning to look like I have a pressure/flow problem, as the new one is misbehaving annoyingly... it has a very sensitive pressure switch, so if you turn the rate down to increase the flow, and lower the temperature, the pressure reduces enough to switch the heat off completely... so you can only ever have really hot showers, even on the low heat setting.. it's even worse than the old Aqualisa! It seems crazy that the pressure switch is working against the possibility of increasing flow rate, which surely is the most important aspect of a shower!
On an adjacent sink, I'm getting a flow rate of 10L/min; On cold at full flow on the Triton, I'm getting a throughput of 6 L/min, and a (relatively) static pressure of 1.7 bar. Maybe I need to rig up something to measure flow rate at the actual shower inlet, with a dynamic pressure reading. (the specs say 1bar at 8L/min at inlet).
Am I just too close to the threshold of the specs? Or have I got a rogue Triton.
I was thinking maybe I should've gone for a lower power model; but this isn't even working on the low power setting, so I can't see that helping.
I'll try ringing Triton tomorrow, but thought I'd see if anyone else had any thoughts in the meantime.

what happens if you take the shower head off, and let water gush from the hose?

All tests have been done without the shower head on John.

phil63 said:

The old shower didn't have a 'cool' opttion, so in summer it was always too hot, even on the low rate option.

Click to expand...

If the old one did something similar then it sounds like a flow/pressure issue rather than a shower problem.

If my loft is on the cusp of acceptable pressure/flow rate, my next question is how to resolve this. It has a fairly straight 15mm supply from the ground floor, unfortunately difficult access to swap out to 22mm. I've installed full bore service valve atleast.
Researching pump options; it doesn't seem to be considered good practice to put a mains compliant 12l/min pump on just the vertical cold run up to the loft: Is there a reason for this? I can't find one (suction/backflow?).. I don't want to alter anything downstairs, or put a pump at the mains in, it's all running fine. But the mains pumps specs. seem to suggest they have to be 'whole house'?? I have a handy space I could easily fit a pump upstairs, 6 feet directly below the shower...