OK. Like most people, I thought that it was definately the emerging driver who was at fault. As pointed out upthread, the responsibility is on the emerging driver to ensure that it is safe to pull out. I also think that the flashing is misleading and that the motorcyclist should be alert and vigilant but those are really side issues.
However... that was not the assessment of the officer present at the scene. His conclusion? Nobody is at fault.
Nobody.
Now forgive me if I'm paraphrasing, I'm trying to be objective and not put words into people's mouths, but I may not be able to remember exactly. What he said was:
I've just got back from talking to the motorcyclist (who was in the back of an ambulance) and he claims it's the driver's fault because he (the motorcyclist) had right of way, and he's technically right, but he (the driver) has got to pull out some time. It's just one of those things. No further action.
@w00hoo_kent I think you called it right

Note OB's use of the phrase
right of way. One for the pedants.
I'm not sure if the motorbike (it was actually a scooter) was filtering. According to the driver
he came speeding down there... Similarly I don't recall if the driver was turning left or right. Might be worth noting they were both young lads so inexperienced.
So basically the lesson for us as cyclists is to be vigilant at junctions with minor roads, because
he's got to pull out some time.