Cheers for the Help David .
I was think of doing this which would turn out cheaper in the long run. the rails in the box look like they have been beaten up slightly and also pin tacked in, I'll probably make a bit more of a mess of them trying to remove them, think its also a good idea to contact Thornes in the link you put up David and see what bits and peices I can pick up.
the trouble I have here is there are no local wood merchants which is a pity as it doesn't look that much to knock up another brood box