Big a dummy question I suspect, but it's not obvious to me how to turn off
the valve to prevent oil coming out of my oil tank.

I have one of these on my tank:

http://www.atkinsonequipment.com/library/atk_p3.pdf

However, on the front, which I assume is the valve, there is some sort of
curved cover (looks like the thing on the _back_ of the valve in the picture
linked to aboe). Originally it was sitting there, but hadn't "bitten" on
the thread. I pushed it in, assuming to turn it clockwise to turn the valve
off (as indicated on the valve). However, it wouldn't bite and screw in
turning clockwise. Pushing it in and turning anti-clockwise it does bite
and screws in. Have I turned it off?

Cheers,
Piers