I came across this yesterday changing the oil on a car with an early apollo tank - I have only seen them before with, as you say, a 32mm hex on the base. The drain plug takes a 10mm hex but luckily it had not been over-tightened previously, and was easily undone.
Hopefully yours will undo easily. Just make sure the tank is held securely, prehaps with an assistant, whilst undoing the drain plug.
Quoting Wrightpayne: I never bothered with draining the Apollo - only a bit left in the bottom and prob more left in the engine!
Ian
I always find at least 2 litres of oil in the apollo, even when it has been standing for a period