Thanks all for the replies.
Flaring the tube isn't a bad idea at all, as long as I can find someone or equipment capable of going that big.
Thanks for the Bunnings tip, but thats still 25mm, not inch tubing that I'm after!
Will try the place in Melbourne and see what they say.
And as Mark mentions, if the lower clamps work at 25mm - great. I can machine the top ball plugs down to fit, but the only issue then is fitting the star nuts inside (maybe grind them down?) - plus - my rings have already been drilled for 1" diameter seats so it'll look a mess. D'oh!
Regards
Rob.