When i went upstairs earlier to get my bike to pop out to the bank, I realised that I hadn't mentioned something.
I found that the bag was sitting a bit low, and, in particular, the rear light wasn't at a very good angle, so I used a couple of toestraps crossed from the Bagman down to the pannier rack, which just sit the bag a tad further out. That's on my Ridgeback, but not any of the others.