Daikin Altherma Freezing up

Hello, first time poster in need of help.
I have a Daikin Altherma LT split. EHBX08CB inside and ERLQ008CAV3 outside. It worked great for about 3 years then for the last 3 winters has been frosting up when the weather gets below about 5C.

I have had 2 engineers out, checked the refidgerant, checked for leaks, regassed and then scratch their heads and eventually the weather improves and repeat the next year. The last guy installed a buffer tank and as soon as the weather turned this year it's done it again.

The indoor unit reports a 89-01 error and the outdoor heat exchanger has about 30mm of ice all over the back. I have been watching it closely and i have never seen it go into defrost??? That is I have never seen the frost on the heat exchanger melt in any way.

This got me investigating more.

Going into the installer menu i can see the warning errors logged and noticed that 89-02 was coming up which appears to be a problem with the 4-way valve not reversing. So last week I took the outdoor unit apart and following the service manual I found online checked the 4-way valve, the coil and the PCB.

I found that the coil measured 550 Ohms in resistance and not the 1-2kOhms that was specified and the connector S80 on the PCB always seemed to read 0V on 220-240V. So I ordered and new Coil and PCB. I have since fitted these and it still doesn't defrost :( interestingly the new coil also measured 550 Ohms so i think the manual is wrong?

While i was waiting for the PCB I did some investigating. The coil is energised but the PCB via the relay MT04 which is a 12V relay. So I soldered wires onto the 12V terminals of this relay. I set up a datalogger to record the voltage on these terminals expecting to see it change from 0 to 12V for a few minutes every 30 mins or so but got nothing. Over about 2 hours never did the relay try to energise the 4-way valve coil to reverse the 4-way valve and defrost the outdoor unit.

As I said I have since replaced the PCB and it still doesn't work so i conclude that for some reason my system never tries to defrost the unit. I have checked the thermistors that measure the outside temp and that seems OK but not sure where the other ones are to test? And I don't know where else to go with this? Why is it not trying to defrost?

Attached are some photos of how icy the outdoor unit gets.
Attachment 16172
Attachment 16173
Please help...