it's difficult to see from above but would be easier if you removed the airbox - should be a plastic 'wingnut' affair on the front of the box (sort of behind the rad)
short length of (clean) hosepipe would do to fill it up with - obv. need to do it with the car level, so don't jack it up
how much oil have you got and do you know how old the stuff that's in it is? it takes 3L for a full change
it's not much bother to revisit it (once you've figured it out), but would be inclined to change it rather than top it up if it's the original oil
ed. typing too slowly, largely been said