OK, time for an update on my research and experimentations.
I'm in New Zealand but the car is effectively a British model which is set to Km/hr so please bare with me on the Km's.
I'm from Glasgow originally and I've been living in NZ, with Kiwi wife and 4 kids, for several years now. Lovely place, visit if you get the chance!
My background - I'm not a motor mechanic but I am a software engineer with a degree in Electronics and Comp Sci, just to explain my assumptions if any.
Car dropped to 1500Km's remaining last night on route home. Seems to drop in 300Km increments.
I went back to the garage where I had filled up the car from the AdBlue pump just to check it was AdBlue.
Yep, it would appear that the only UREA emissions additive they have here in NZ is just one type of "AdBlue" which is used in all cars.
I then bought a 20L jerry can and some piping and syphoned out as much of the AdBlue out of my car as I could managed.
I got just under 15L out. When I filled it up at the pump it only took 14.3L to fill it. I'm now wondering if in fact my car has a 15L tank.
Before I did any of this I had used the "Car Scanner" App with a WiFi OBD2 dongle on my car to check for fault codes. None found.
When I looked in "Noncontinuous Monitors" I discovered one fail. "NOx Catalyst Monitor Bank 1 - MID$98 Test ID $B1" ($98 & $B1 I would guess are hexidecimal addresses).
Now with the AdBlue tank empty we went on a long Sunday drive to my mother-in-laws. It was her birthday and it's also mother's day here in NZ.
That would be about 100Km round trip.
After this trip I plugged in my OBD2 dongle and checked the errors codes. Still no ECU errors but the "Noncontinuous Monitors" now have 2 errors.
The new error being "NOx Catalyst Monitor Bank 1 - MID$98 Test ID $B3"
When we got home last night I filed her up with 10L from a new bottle of AdBlue I had bought - incase the stuff I'd got at the pump was tainted/wrong.
Left it alone all night, not opening doors, turning it on etc etc like the instructions tell you.
The wife took it for a drive this morning and when it came back it still had the UREA light on.
So I hooked up the OBD2 dongle and took another reading.
It's now back to just 1 error code and the reading 32.766 has never changed. It's been at that value ever since the problem started.
The other NOx sensor now reads 47.2 which is well within the manufacturers threshold of -50> <75
Wife took a very short journey so I'm not too concerned about the higher reading of 47.2 compared to 3 originally.
I'm reading these results as an indication that the AdBlue tank/pump system is working fine but that there's a NOx sensor issue.
I'm going to check in with my local garage to see if they can do anything before I take it to the dealer which is much further away and expensive.
Thoughts/opinions?