Aqualisa Axis low flow problem?

I have an Aqualisa Axis that is just over 3 years old. For most of the time it's been faultless but a few months ago it started to come on at low pressure and about half the time would pick up suddenly and instantly to full pressure after a couple of minutes. Now it's always low pressure - good enough to shower but not a 'power' shower experience - more like a half decent electric shower.

We've had an engineer out to sort this out plus another issue with the lights staying on (which he fixed).

He was very reluctant to look at the pressure, immediately insisting it could have nothing to do with the shower which he says doesn't regulate pressure and is probably the magaflow boiler or us not cleaning the head enough. I did push him on it though as I have seen (though can't now find) other people writing up similar issues.

He says he measured the flow out of the shower at 22 litres per minute - the max for the shower and said that meant the low flow problem was with the megaflow and that we needed a plumber to regenerate the air gap. The rest of our houses water pressure hot and cold is very strong. I did think how can he say there is a problem elsewhere if the shower is allegedly at full strength (which means it must have been pumping out twice as much as it was meant to for the previous 3 years!)

We live in one of a newish pair of semi's and our next door neighbour has the exact same set up. Their shower still stings your hand where ours doesn't get close. I just think that we've been fobbed off but I'm not sure where to go next. I will regenerate the air gap following the instructions on the boiler but this was done during a service 3 months ago.

Has anyone experienced similar?