Arthur has suffered some damage to his oil filler cap, here is a pic of his cap on his rocker cover BEFORE it got broken:

I've spent the morning chasing around locally in someone else's car to try to get hold of it, but no one who's open on a Saturday morning seems to stock it, alternatives they offered don't fit/seal in my rocker cover.
Two questions for you lovely TMFers:
1) is it ok to drive without it a) for a short distance - say 5 miles b) for any other journey? (and an explanation of why/why not would also be welcome, ta)
2) I think the correct replacement is this one from minispares, am I right?
Red rocker cover oil filler cap on minispares website
I don't want to pay a fortune for speedy delivery only for it to be the wrong part
Thanking you.