Thanks to all for the advice above. Was finally able to get Techstream 18 installed and running (although this was not a straightforward process at all, with plenty of driver issues, and virus scares along the way)
Running diagnostic check on all the ECUs:
Lots of ECU faults! Some historical, some current:
There's probably a way to clear the historical faults but I didn't have time to figure it out.
The following two screens helped me to finally diagnose the issue:
Points go to @IS300FSPORT - it turned out to be an issue with wheel speed sensor.
However - there is an ironic backstory to this.
A while back, I replaced the suspension strut rubber gaiters - and like a true novice, I forgot to unplug the ABS cable before disconnecting the upper control arm and... SNAP! There goes the ABS cable. I soldered it back together and sealed with electrical tape. Clearly this bodge job has come back to haunt me! 🤣🤣🤣