(assuming 0% commission at Smarkets or Matchbook via OddsMonkey or Profit Accumulator).Not even! Looks like straight cash refund! How do you calculate these ones?
Various tactics, for example:
Back £20 at Skybet on horse A @ 3/1 (4.00).
Lay £20 on horse A at Smarkets @ 4.00.
This is a hit and hope strategy with no qualifying loss if you find and exact back and lay odds match.
Or thinking about it a bit more, it's a refund on lose offer so you wanna underlay at the exchange to maximise the amount of any potential refund.
Top of the head calc's: Back £20 on Skybet @ 4.00 (potential win £60), lay £15 on Smarkets at 4.00 (potential liability £45.
If horse wins you gain £15 (60 minus 45), or if lose get £20 cash refund.