That's for the reply. For mounting the servo itself I was thinking of inverting it and having the rotational centre directly above that of the door to keep things straight forward. The servo I linked to was a 13kg load one, does that sound reasonable? The resistance to move the door would be less than it takes to turn a doorknob I'd say so hopefully 13kg will be enough.
Please excuse the proably stupid question but I've not seen the term vin mentioned, what is that?
Would I need to split power off some way to the servo or would it just be a matter of plugging the power into the arduino board and taking it through to the servo?
I'll add a RTC clock to the shopping list. I've seen various starter kits that include connectors and cables and various other bits and bobs. If there was one that came with a servo that'd be handy - at least I'd know I had everything I need.