I've actually just done this with a probability tree, and I worked out that to hit the £400 target, you would need to win 3qty £100 bets in a row, therefore the probability of this happening is (I think)

48.6% x 48.6% x 48.6 = 11.51%

However this assumes that all three bets win. Does this probability change, for example, if you were to win a bet, lose a bet, then win etc etc.... ??