as above is it possible to remove the extension and if so what oil cap is needed?

Yeh, you just undo it like your oilcap, and I am pretty sure you just use the one you have.

My extension always comes off when I undo the oilcap itself.

Standard oil cap will fit straight into the cam cover and seal ok, but when shut it sits the opposite way to standard.

is there any other oil cap options that sit straight? what about the Porsche 1 as that sits to the side with filler extension