I would never recommend flashing for the front. Indeed none of the front lamps I currently use has a flashing mode. Especially on an unlit road, where your front lamp is primarily for you to see by, and you cannot see properly in flashing.
For the rear, opinions vary. I use flashing when riding on my own: I was once told by a motorist who had passed me, that he found my flashing light made far more impression on him than a steady one. And being noticed can mean your life saved! Some disagree with this (with the flashing idea, I mean, not with having your life saved!)
Regarding the comment about Dunwich: perhaps for a large group ride, it should be down to the organisers to stipulate whether rear lights should be steady or flashing. Does Simon have any rules about the FNRTTC? (last time I went on one, most rears were flashing and I agree it was a little bit off-putting).