Fella, I know you put this on as a fun bet and your exposure may have been tiny, but you could have laid off the some or all of the bet on the in running market on Betfair.
I did not check the lowest price it got to in running, but it must have got lower than 10/1.
Often on an arb my strategy is to lay a horse for no loss if it loses and decent profit if it wins, then put in a lay in running at much lower odds to get some profit if it runs well and still loses.
The bummer comes , of course, when you actually fancy the nag to win and it does.
Anyway I am sure I am teaching you to suck eggs as you are plainly an experienced punter
If I'm just mugpunting for fun I do tend to leave some in-running lays however they'll usually be at very short odds (i.e. insurance against a very near miss). I think I had £100 @ 2.0 & £200 @ 1.5 on the win market (obviously neither of those were triggered, I think Golden Sword may have gone down to about 6.0 in-running but nowhere near evens).
Probably more accurate to describe me as an experienced Mug BTW