Responding to Dan
Hi Dan, it seems I can't reply to your message because I haven't posted often enough in this forum (I'm new here).
The basic principle is as follows: As a promotional offer a bookie gives you a 50 GBP free bet if you bet 50 GBP of your own money first. So you bet with your own money at odds of 2 that Team A beats Team B in football. Then you go to a betting exchange such as betfair and there you bet that Team A won't beat Team B (this is called laying a bet). So if Team A wins, you win 50 at the bookie, but lose 50 at the betting exchange. If Team B wins or if it is a draw, then you win 50 at the exchange, but lose 50 at the bookie. Whatever happens, no win and no loss (this is why it is called matched betting, it is risk free as opposed to betting). But now you get the free bet from the bookie and do the same. If Team A wins, you win 50 at the bookie (so you have 100 there, including the free bet stake) and you lose 50 at the exchange. In total you win 50. If Team A does not win, you lose the free stake at the bookie, but win 50 at the exchange, again a profit of 50. This is a simple example, in reality it is often less easy. But the same principles applies and stakes can be calculated with an Excel spreadsheet (which I can give you). After that you'd move to the next bookie and do the same again (generally these offers are for opening an account).
If you want to send me your email address via PM I can give you the spreadsheet and a link to a website that explains everything in detail (they don't like publicity so I don't want to post the link here).
I know it sounds a bit dodgy, it took my colleague 3 months to convince me ;-) But it does work.