[Talonmotorsport]

When the car was mocked up as a prototype I'm guessing that the materials used where what Martin Keenan had laying in the steel rack at the time. The roll bar in the book is 33.7mm with a 3mm wall as it's the same size as the suspension bush tube material so I'm guessing he had alot of it at the time.
You don't need CDS for the roll bar stays ERW or even HRS will be good enough as it's the roll bar hoop that takes all the punishment.
The roll bar material can be any thing from 30mm dia. 5mm wall up to 75mm dia 2mm wall, it's your car you can make it out of what ever you want. You could use a scaffold pole if you have a way of bending it, mind you there is nothing to stop you making it 5 bits, welding all together and lobbster backing it.
You don't need to use CDS unless you intend on racing the car as bare in mind the chassis is only 1.6-2mm any way , I would say 2.5mm wall min. would be the thinest I would go.
I've built fully space framed grass track cars from 32mm 2.5mm wall ERW and they hit telegraph poles at 70mph and are still drivable afterwards.
The book is a guide to building a car not the absolute rules.