If players could go online and bet - because I guess they wouldn't [if they could] walk into a shop - that e.g. 'Daniel Sturridge on loan to West Brom', they'd clean up. There's got to be some jiggery pokery / spin about what's actually happened.
The problem is they get caught setting up the account. theres cases of players getting bans for doing that and betting a few quid and being flagged. I think andros townsend example was one i gave who did just that.
The only means for the FA to be investigating is a complaint is made by betting firm who don't like paying out. They have had to have had enough data to show what happened. The FA could have spend months looking at evidence of betting pattern and interviewing sturridge as to do you know this person or this persoin etc etc.
As he is now charged one can only conclude that an odd pattern of bets occurred on some event in january that related to sturridge (we assume we know what that is) and some people made money on it that are able to be associated with sturridge.

