This was funny, but a little hard to follow—maybe because of the sudden appaearance of Mark.

The “an” in I’m an RP nerd is unnecessary, it’s just “a”.

I think paragraphing more clearly might help with readability, but you’ve got some really good dialogue here…

Personally I think the ‘an’ should stay, that sounds more natural to me.

Reading back over things and checking that somebody who’s never seen it before will get what’s happening is a good way to spot any points of potential confusion i.e. the introduction of Mark.

But overall, good work, particularly for a first story – welcome to Ficly! MH :)