If I multi-quote we will be here all week, so...
Thanks to everyone who has posted so far. I'm glad my good fortune has been enjoyed by so many other people. Absolutely a once in a lifetime experience, and it absolutely blew my mind. I still can't believe I was that lucky, and had the camera with me as well.
For those that asked, the little snakes (snakelets? snakelings?) are fully independent of mum as soon as they are born, and she leaves them to fend for themselves. I was hoping Jonathan D would drop by and tell us all more, since snakes don't really have enough legs to be my area of expertise.
I bet if I go back there for the next year or so I won't even see an adder. The site is really densely vegetated, and this was one of only a few patches of more exposed ground. On the other hand, adders like to bask, so now I know where to look.