I would say that if you take 20 trades you can expect to win 1/20 (5% change) times £1000 and loose 19 times £100. So you will probably loose £1900 and win £1000. So this will give you a loss of £900. If you win 19 times a £100 and only loose £100 once you'll end up making £1800. I'll go for the £100 gain with a probability of 95%.