I think I just sent a message to the wrong player (don't I feel sheepish) so am curious, if I delete it at my end (presumably before the player sees it) is it gone from their inbox, or is it a case of it just clearing it from this side of the account but it still remains on theirs?

Yes, so that means, they will still have the msg in their inbox.

that's what I thought, thankyou so much for clarifying for me though