Ross, it is comon knowledge that whitbread cages are made from blue band (its all they used to be able to bend). As for my authority I dont have any, it is writen in black and white in the MSA blue book (have you checked yet)
As i said in my first post there is nothing wrong with Whitbread cages for what they are ment for.
I think Jericho is right, NOR is ok good value for money, PP you get what you pay for and dont forget Tornado Motorsport they do very good cages too.
Shox Dr I think you will find that CFS360NBK (Hydraulic line tube) does not meet the MSA yeild strength so if you are using it for roll cages i sugest you find out what blue band and clubman 500 are!
Also try Exact fabrication, Tornado motorsport (www.tornadomotorsport.co.uk) and Protection and performance ( www.ppcages.com ) PP cages have some really cheep kits on Ebay.