Hi all, after much probing, debugging, etc, I'm pretty sure I've worked out where the fault with my front wipers. It appears the BCM is unable to output 12V on T73a/71 or /69 (fast or slow wipe). I suspect there is an issue with a blown FET or something in the BCM. There is no issue with the stalk or the fuses.
I've opened up the BCM, and have inspected the outputs on T73a/71 and /69. They are definitely relay selected (OMRON G8NB-17SR), and as such I'm doubtful the wipers are controlled by PWM. Also I can energise T73a/71 and 69 with the loom unplugged and get the wipers to move.
How I suspect I damaged the device was by trying to apply 12V to the wipers directly with a battery - which turned out to be flat. It was only slightly activating the wipers, and potentially creating a problem on the BCM. I may have also had a dodgy ground which could have been creating problems.
Anyways, unless someone on the site has a schematic of the BCM circuit board, I'm probably off to the wreckers to get a new BCM.
Moral of the story: Use a proper vehicle tester probe to energise circuits on the T6, rather than my ghetto battery circuit. Probably also don't let electronics engineers without much vehicle experience near your own vehicle!