Not just using your hands on a small allen key are you? Need a long allen key and a long extension tube on that too. Preferably something you can smack with a good hammer.
But if all else fails then use a mole wrench with curved jaws around the outside of the drain plug. Make sure it's on good and tight at 90 degrees to the plug, level with the base of the sump and hit that
got the drain plug key on a wrench so i should get a hammer a give it few good hits?
Don't like hitting wrenches so i'd use an extension tube directly on a big jobby allen key
I don't need to touch the rubber seal bit at all?
No
The way to open it would be to turn the wrench to the left?
Anticlockwise viewed from below
or go for plan B with the mole wrench, it's too cold out there to be messing about