Mitsubishi Ecodans ASHP issues...

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Evening all, would appreciate your input and advice on the following issue from another forum: As usual I know your the best to ask and I have asked the op to sign up on here so any questions can be asked, I have quoted a few posts and replies etc.

There are also pictures of the unit included:

Hi guys, new to the forum, and please bear with me if I ask something here that's already been asked, I read everything up to post 830 and thought I'd skip ahead for some advice

Ok, quick run down of my install:
2 x 14kw EcoDans heating a 170m/2 house with 31 radiators, mostly doubles (!, yes I know), if you count a hall or a landing as a room then 17 rooms, no buffer tank fitted (as Mitsubishi say no need),
2 x 210 litre DHW tanks,
28mm, 22mm and some 15mm pipe, 4 x 15/60 Grundfos Alpha pumps (2 pull, 2 push).
No seperate zones, most rads have TRV, but all turned to "max on",
400mm loft insulation, 600mm solid stone walls uninsulated due to listing. Full double glazing.
SAP report said 15kw required, and walls have a U value of 1.9.
House is undergoing refurb and not lived in. Heating was set to 10'c, water from Ecodans 22'c @ 17'c ambient ramping to 38'c @ -10'c ambient. No DHW generating set at the moment.
House is an old croft in northern Scotland (KW11 6UA, near Altnaharra, you may have heard of it recently) Recent temps dropped to -16'c and hovered around -10'c for several days.

Last winter the units iced up and failed due to inadequate water flow. Replumbed and sorted that. Both Mitsubishi and the install company are now happy with the water flow. Last winter it got down to -22'c, but the heating was off due to the failures.

This year they have again iced up to the point that the air flow is completely blocked. They try to defrost but fail, go back to heating and build up more ice, try to defrost etc....
Honestly, the ice was a solid sheet 10mm thick covering the back and sides of both units, last year it did this so thickly the fan blades sheared off due to the build up. Caught it this year before that happened.

When we're there and the room stat is set at 18'c the Ecodans easily manage to hit target temp within an hour of a cold start (31 out, 31 return) and the house is up to temp in about 3 hours, so I don't think it's a problem of sizing. They get the DHW tanks up to 58'c in 30 mins when we need it.

When they do a defrost it can be as frequent as 10 mins apart, or as long as 45, I'm sure they've never run for 2 hours without one. It lasts about 2 mins, though I haven't timed it. Interestingly Mitsubishi website says 30 to 120 mins apart, definitely more frequent than that.

When it fails it's different: it says "defrost" on the indoor unit, the outdoor shuts down, then you can hear the compressor start, then it makes a huffing sound and stops. The indoor still says "defrost" but the outdoor is doing nothing. After a couple of mins they then go back to heating mode and the cycle starts again.....it doesn't throw any codes or warnings on the indoor units, and I don't want to take the covers off outside to check as I don't want them to say I've voided my warranty by doing so. Sometimes after several failed defrosts it does a good one and then dumps loads of water out. It seems to fail more over night than during the day for some reason.

While the failures are bad enough it's the electric consumption we're more concerned about. To keep the house at 18'c (overnight 14'c) when outdoors it's ~8'c took roughly 100 kwh / 24hrs. At our rate that's about £11. When people nearby in similar houses with similar insulation are spending about £6 / 24hr heating to 22'c and DHW we're more than a bit concerned! In the temps we've just been having I imagine it would have been substantially higher. At that sort of rate we'd be better off burning £5 notes to keep warm

The only thing I can think of is high humidity causing very frequent defrosts using more electric and dragging the COP right down as a result. We share the valley with a river and according to my weather station thingy humidity is always around the 90% area outdoors (30% indoors). Other than that I can't see any reason why our system doesn't work as promised, and neither can the engineers who have been.

The install company has passed all the info I give them to Mitsubishi, data from my weather station showing indoor & outdoor temps, humidity and wind speed, loads of photo's, and all my observations, and all they said was basically "they're working within parameters, stop bothering us, not our problem".
Real helpful.

The install company is baffled and we're getting a bit worried about our £14k system. I think the next thing they're going to try is to put a buffer tank in but I don't see how that's going to help.

The main problem as far as I can see is the failure to defrost. It's not the fact that the units defrost but fail to melt all the "frost" on them, when they do a succesful defrost they clear completely down to the fins.
It's just that sometimes they try to defrost but fail to complete the cycle, actually, they fail to start the cycle. The indoor unit says "defrosting" but the outdoor unit is completly shut down, doing absolutely nothing. It tries to start the defrost, makes a huffing noise and then totally shuts down. The indoor unit says defrosting for another two mins or so, then kicks back into heating mode. This then sucks more moisture onto the exchanger and it builds up a sheet of ice.
When the exchanger is completly covered, it keeps the ice well below freezing by trying to extract heat from it and any moisture that touches it then adds to the thickness.
Even if a succesfull defrost occurs it leaves a sheet of ice clinging to the frame over the exchanger but not actually touching it so it cannot be defrosted by the unit. This in turn leades to no airflow and extreme electric consumption. Add in ambient sub zero temps and the ice simply stays put. Mitsubishi initially said it's wind chill, there was no wind this time, and the ice was only on the heat exchanger end of the case, so I don't think that's the cause.

Thank you for reading this much, sorry, it did get a bit rambling in places, I tried to keep it short while putting all the info down