I'd really like to extend my thanks to
@jacksparks for resolving this for me.
Having reviewed my account history he has confirmed that the bet was in his opinion clearly placed in error. (As I mentioned earlier, I've made a few deposits/withdrawals from 21Casino and have a fairly chunky game history with them, so he has a decent amount of data on my playstyle.)
As such he has credited £125 to my casino account to put me back in the position I was before placing the unfortunate bet!
Obviously there's no kind of precedent being set here and the casino had no obligation to refund me anything, and I'm certainly going to be more careful in future because I wouldn't expect to be extended the same courtesy twice.
But yes, many thanks to Jack and a very good advertisement for only playing at accredited casinos with reps active here at the forums!
Also as per Jack's post above, he's going to feed back to the provider about this, as it's not a great piece of UI design!
Anyway, a happy resolution in the end