Fantastic! Cheers for the info guys.
After that I will definately be looking into other bearings, seems like its almost the norm to improve this (I have noticed the poor quality of the old bush). And yes Deezee, that's the bit I meant!
My main reason for wanting to use the roof for the floor is to keep costs low. and large sheets of Ali are a little on the pricey side
I'll keep my options open on that one for now.
I am a Mechanical Engineer and the only reason I can see for ERW beeing bad is for its reduced fatique resistance due to the grain hardening caused through the application of heat (Harder = Brittle).
Around 2.5mm wall thickness sounds about right, need to double check what I had in my notes (when I find them...)
Thanks for all the advice so far guys, I'll think some more on it when I'm more awake!