Hi Nigel
Im afraid Im a bit late in seeing this

but I thought I would post anyway.
We too have had a pair of BFs at the feeder since early summer, they then appeared with two youngsters. The pair have hung around since but recently I think I am only seeing a female - it could even be one or both of the youngsters. I havent seen the male for a couple of weeks now, so I dont know if the male has moved or or if the others will move on.
I didnt manage a photo though as I didnt dare go out there with the camera because like you say, they were very wary.
By the way, I had flowers on the Valerian this year

(but no hawk moths) - still havent seen one lol.
Linda