wow, thanks for all your great replies.
I was thinking of the best canon 50mm lens to reverse, but by the sound of it i would be better off saving up for the MP-E. I was just after a cost saving really.
I did just get today the proper canon flashgun bracket (i know stupidly expensive for what it is), and i can say, what a lovey bit of kit tied with my canon 2.8L 100mm for 1:1 macro, even with no diffuser im getting some nice results (albeit only 1:1)
I would be interested in the Kenko tubes but for £100+ to go from 1:1 to 1:68 i cant see the value.