Shaun, yes I was aware of backing with a bookie with a minimum price stipulation, in fact I've done it with some success, but then the mob I was dealing with decided they would refuse some bets and not others so it killed it.

What I like the sound of is being able to do it mechanically, (i.e. if the price is met you're on if it isn't you're not) so I'll be in touch later......... thanks for that!