Mira Sport Max 10.8 kW very low flow?

I have a low flow problem with a Mira Sport Max 10.8 kW

A little background:

Recently the shower began to shut itself off without warning. Other than that it was perfectly well behaved. Then it died. When the isolator switch was turned on the red led lights behind the buttons would come on, but nothing else.

After checking the inlet valve I tested the solenoid coil with a multimeter and got a reading of 2.7 ohm, ie useless. In taking it off to replace the bottom of the flow valve assembly came loose, so I replaced it which was very straight forward.

However, I’ve not been able to commission the shower since. Starting at cold on low the flow was already lower than expected. By the time the temperature dial is ¾ turned the flow is just a dribble.

The shower is much louder than before. There’s a loud, dull roaring like a distant waterfall, much like a radiator that either needs to be bled or the flow rate is too high.

Turning on the medium flow button on the coldest setting caused the low flow light to come on within 10 seconds after which the shower either shuts off or turns itself to the low setting, which is not something I’ve ever heard of before.

Of note:

We have extremely soft water.

The stop cock is fully open, there are no other valves between the stop cock and flow inlet valve.

I have tried commissioning without the shower head and the hose in a down position.

There is nil risk of any back-siphoning up the hose.

The shower parts are all date stamped 1998, I think it must be one of the first Mira Sport Max units.

Has anyone come across this before? Is it fixable? Am I missing something really obvious?