Theory

In theory trading on as many profitable horses as possible during the day will give best results. What determines a profitable horse is the problem I am solving. In order to find these values I am considering the following variables.

Stake - the amount to initiate the trade - I suspect that the market will only tolerate small amounts for a reliable profit. i.e. you are more likely to get £10 matched immediately than £100.
Odds Limit - the max price used - using large prices means you need a lot of cash in your account to support the Lays. (e.g. £10 lay at 40 requires £400)
Max Profit Per Trade
Max Spike - this is to cater for the large spikes in the movement.
Max Number of ticks a market can move before being closed
Max Profit that can be made before a market is closed.
Minimum Amount available for bet
Minimum amount matched on that runner.

With a profitable set of parameters, trading on 1-3 horses per race should give approx 0-£10 profit per race, using simulations this seems to give about £20 per day (about 12 races). Not a lot for the amount of activity required, but not bad for the push of a button!

To simulate I basically used a stake of £10 on every race for horses with a max price of 5. The bets were not actually placed, but the theoretical profit was calculated given the live prices on the market.

The prices were captured and fed into the bot with a set of values for each variable. The results were then analysed in an Excel spreadsheet to determine the set of values that gave the most reliable profit. I have been looking at 10^8 results - this required me to buy a new PC as my old single-core machine was creaking at the seams!