I've got a light fitting in a shaded area. I need to fit one of these
screw-in photo-cell adaptors to it. Does anyone have any recommendations
about models to look at / avoid. In particular I need to be able to
adjust the sensitivity of the unit.

It's a waste of electricity to use a light sensing switch in the socket, as
it leaves the light on all night. Why not get a motion detector switch, and
connect it to the fixture, or just a motion detector floodlight fixture
which would likely be as cheap or cheaper than the switch unit.. The good
ones have light sensor adjustments as well as choices of time-on.