BetFred

I decided to trust BetFred as their credit/debit card entry form is secure, although they would improve customer confidence if they didn't hide that fact by putting it in an IFRAME.

I deposited my £50 and bet on there being more than 2.5 goals in the Slovakia v Northern Ireland game at odds of 2.1. A Betfair lay of £48.84 at 2.2 meant that a 2 : 1 Slovakia win resulted in a £55 win with BetFred and a £58.61 loss with Betfair which is a loss of £3.61 meaning the profit is down to £59.17.

I got the free bet 24 hours later and bet on the match between Iceland and Scotland being a draw (match is on the 10th September). This had odds of 3.25, so the £50 free bet will return £112.50 if it wins. I had to lay twice with Betfair as the odds I wanted didn't have enough money behind them. I laid £20.40 at odds of 3.4 and £12.98 at odds of 3.45. This means that I will make a £31.74 profit if the match ends in a draw and a £31.71 profit if it doesn't. I will update the overall profit after the event.
Update: Scotland won 1 : 2, so my profit from this free bet is £31.71. My overall profit now stands at £90.88.