Those aren't mug bets though - you're taking too much value.
Make a few of those regularly and the bookie will ban your account.
Oh. That was one lucky occasion though, i tend to do lots for £20 where the odds are just slightly out, making a quid here and there, rather than just bet on MBS, especially on Skybet etc where you have to stake an amount per week for a free bet.
So @amigafan2003 makes a good point - bookies will cotton on
But that aside, what stops you from extending your theory the 18/1 shot won't win? Why not just lay the horse without backing it - it'll never win right?
It's not that extreme, it's breaking even worst case but rather than win a quid on my little bets i win £1.50. it's little bits but they soon add up and if the maths works why not? Obviously amigafan has made a good point as to why not.
The other reason I'm doing volumes of bets is the sites where i got a massive sign up bonus, but need to turn it over.