So as this happened yesterday and coincidentally, they had the part...I have it back now. Super quick turnaround, which is great.

Urea Injector was the culprit. It was broken due to "brittle plastic".

Parts used to repair:

18-30-7-807-206 Meter Unit
83-19-0-441-139 Diesel Exh. Fluid

It still has some of the ammonia smells, but they say it will burn off in time.

X5 back up and running as normal again.