Neil, I go out to the Isle of May every year at least once, but much earlier in the year, usually in May, when the pufflings have hatched.
Of course, you don't see the pufflings, as they are deep in the burrow (or the gulls get them!!)
I've played with various combos, and find that, as the birds are quite close (although you can't go off the path) 300 or 420 is plenty, and even for the static shots, you'll be fine.
My (albeit limited) experience of the 800 is that the field of view is so narrow, you'd be extremely lucky to get a puffin in flight with it, that lens is good for static shots but in flight?....well perhaps its my cr@p technique but I just don't think it would be any good.
You'd be much better with a 5 or 600, imho.
Will research the dates a bit more, and perhaps pm you.
George.