Ok, quick note for you Gord (you haven't done anything wrong, this is just to help you streamline things a bit). You say you deposited £100 into Betfair. Let me focus on that for a moment.
I've picked this bet because it meets the odds criteria of 1.5 minimum and is on a fairly well known game (don't go betting on weird absurd stuff you've never heard of, this is an instant flag to them):
As you can see in the liability section, the liability for this bet is £5.39, which is what you'll lose if the back bet wins. With that in mind, I would've deposited no more than £20. The reason being that if the lay bet wins, you have more in your exchange to lay bets off, and more in your bank account for more offers.
If your bankroll is £200 and you win a few back bets, you're screwed and have to wait until some withdrawals clear.
Just thought I'd throw that out there to help out