I've been reading some of the boards and most people think he bet on every horse on the last race. Yes, she did win, however, it wasn't the horse that Darren told her would win initially. After the race he said look at your ticket and apparently it said that he bet on the other horse, in fact. Someone on one of the boards was saying that the tickets don't actually list the horse's name but just a number referencing it so it could have been any horse.
Even betting £4,000 on every horse would be peanuts compared to the budget of the show and how much they make. Placing a bet on every horse actually fits in with the theme that there is not actually a "system".
I'm just not sure about the bit with the horse trainers/journalists who had to pick the pictures from the room and that Darren guessed them correctly. Did he get in hundreds of journalists and film all of them until he was able to make 4 lucky guesses? It kinda fits in with the idea of the flipping coins and the "system" or did he use his "mind powers" to deduce it?