Thanks for responses.
It's a lot easier getting back to Stansted on the train than relying on a bus (especially the traffic around Victoria during the day).
I can buy a ticket online for £23.40 return for collection at the station (as opposed to £31.50 in the station). It's just I'm arriving quite late and if I get delayed an hour or anything (which happens) I'll miss the last train so was just debating whether to risk leaving it until the plane or not... the tickets are valid on the National Express Coaches if needs be that night anyways. It's just the trek down to the train station to get the ticket would be the balls