I have done numerous of projects based on the betfair API (free and paid version).
the lay odds will be pulled from the betfair API and the fixed odds will be directly grabbed from the betfair website as they are not present on the API.
If you want, we can include odds from different bookmakers -williamhill, betfred, ladbroke- etc... as I have already a grabber of these bookies, this will give you more arbitrage opportunities.
when an opportunity is detected, you will receive an email or an SMS.
If you want so, we can implement an automatic bet placer on both sides to garantee a sure profit whatever the outcome of the race.