Update...
From speaking to a mechanic friend up in Glasgow, he wagers, though cannot check until it's up on a lift, that it's the caliper.
He says he had a very similar experience with an A4. Although counter-intuitive, it ended up being the caliper.
We've worked though everything and Occam's Razor...it seems to be the cause:
1. Intermittent juddering when braking (from front left wheel)
2. This was happening last year, and went when new disks/shoes were fitted. (same side)
3. Owl-like long hoot/howl from same wheel when driving slowly, goes when you just touch the brake as softly as possible.
It all makes some sense, as the noise is definitely a resonation.
Now, I would have thought the mechanic who did the front disks would have looked at the caliper, but...
So, advice here from members is - new calipers or rebuild kit. And if new calipers - I was looking at TRW.
T