Ben - two of my old mainsails in the shed at the club that you can have. I was trying to get hold of Rachel Keeling with no sucess to find a mast. Also try
http://www.fireball-dinghy.org.uk/ - suggest you contact Harvey French in the For Sale section - old Fireball with two masts up for grabs, he might let you have one of the masts for a knock-dwon price and he's in Kings Lynn.