Setting up a free matched bet

I used the Portugal v Turkey game today to set up a matched bet by taking advantage of an online bookmakers signup offer. I deposited £20 in my new account and bet this on the game being a draw at odds of 3.5. I then lay against the game being a draw on Betfair at odds of 3.55. So the possible outcomes were:

As you can see no matter what the outcome of the game I have lost £1. Whilst this may sound like a stupid way of betting as I am guaranteed to lose money, it allows me a risk free way of qualifying for the free bet.

Once the bet has settled I should be credited with a free £20 bet in my new bookmakers account. Then using this same technique again but this time with the free bet stake I should earn a profit of somewhere between 50-75% of the free bet stake.