Yes. You heard me right. Let's all say it together, shall we?
One hundred and eighty-seven P.O.U.N.D.S!!!
Fortunately, the nice people at Xbox support (0800 FML!!!) were extremely sympathetic and well accustomed to witless parents phoning them up in a panic because they'd just realised their precious offspring could spend actual real-life money inside the Robux game.
They kindly gave me a refund with the understanding that this was a one time deal. If this ever happens again, tough titty.
Needless to say I have removed my card details from Microsoft's data base, and am now preparing the wee talk I'll be having with my boy when he gets home from school.