Thanks for all your replies everyone. I'm still trying to work out how to share the webcam footage - at the moment I have a clip but it has no sound when I pull it off which isn't very helpful.
Well last night, I left the crates uncovered at the front and put the webcam up on a chair so I could see both dogs, but it wasn't at eye level so not likely to disturb.
Put them to bed as normal except for pulling down the towels over the front and waited. They ate their biscuits and Angus laid down peacefully in the middle of his cage. Epic on the other hand didn't settle, for about 30 secs he turned around, stopped, looked out the front and then suddenly ...the snarl. NOT at Angus - but at something out to the front of the crate. Angus predictably jumped up startled. Then he settled down again while Epic began the growling. Again out to the front right corner of the crate, not in Angus' direction at all. His (Epic's) muzzle was through the bars and he seemed fixated on something out in the room.
I think that when this first happened, Angus understandably reacted to Epic's snarl but now it's happened a few times he's getting used to it, but this really doesn't seem to be an issue between the two dogs but something that is suddenly bothering Epic when he first goes to bed.
Nothing at all has changed in the room, the routine or anything that I can think of, so why he is suddenly having this weird ritual I've no idea. One thing that possibly suggests itself to me is the worrying thought that there may be a problem with his eyesight, and the light going off is causing a vision issue.
I know 'night blindness' is an early symptom of PRA - but I just conducted an unscientific test of turning out all the lights and having him recall around tables and things and as far as I could tell (as I couldn't see either!) he had no problems at all. Also I would have thought if he was 'seeing things' then this wouldn't happen with the covers pulled down
So I'm further forward in that I know what is happening, but why is still a mystery. Standing in the dark watching it all playing out on the webcam it really does make the hairs on the back of my neck stand up...it is quite unnerving.
Tonight's experiment is to leave the light on low....to be continued...