Shaunymog you must have different scaffolding over there? UK scaffolding tube is certainly not safe to use as a roll cage. Yes it would save you if you gently rolled onto your side but it is not suitable for a rollcage.
I would look at using 'blueband' steam pipe it's approved for landrover cages, can be push bent and is cheaper than cds/ cost effective.
If your tying your cage to the cab make sure you add some floor level tie bars and endure seats and seat belts are bolted to the cage also.
Will.