Can anyone suggest where I might be able to get a mechanism for opening
and closing a small door. The door will probably be about 12" square and
can be hinged so that it swings like a full-sized door so the effort to
open and close it should be quite small.

It needs to be secure against small animals (foxes etc.) when closed.

I want to operate it electrically so it can be opened and closed at
set times (with a time clock from the mains) or possibly be driven by
a photocell.

Searching for door opener only produces electronics for door latches
and/or garage door openers.

--
Chris Green