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Mark is farrier at the raceyard of trainer Evan Williams, who is coaching him and providing his mount. Mark's winnings--if he can beat the pawnbroker, the bricklayer, the fireman and other non-jockeys--will go to a Welsh children's charity cancer.
The race sponsor produced this video about the selection process and followed the green jockeys through their crash (sometimes literally) course at the British Racing School in Newmarket.
You can place a bet at Ladbrokes OTB online; click through sports to racing to Grand National and then look for "Peoples Race." I'm not sure if you have to be in the UK to bet through Ladbrokes or not. The race will also be shown live on the BBC's telecast of the Grand National.
Here's a link to the latest reports on Mark's campaign to become super-fit and psyched to win:
http://www.sportinglife.com/racing/news/story_get.cgi?STORY_NAME=racing/07/04/11/RACING_Peoples_Galliers.html