Not suprisingly, I believe this is a good decision, well done gents.
However.... (I know some people are never happy!!

)
I am sure you have given this a lot of thought, but I don"t think creating one additional £75 prize place per alternate is the best way to go.
For future events, I would like you to consider making one additional prize, of £225, for each 3 alternates. I think most people would prefer one additional chance to pick up a prize that would go some way to covering their expenses for the weekend, rather than 3 chances of just getting their buy-in back.