I stumbled across this
Dave March investigates the technology used to produce the chassis for the Caterham Seven
I find it interesting because I'm using the mcsorley plans with modifications regarding the removal of tubes in the chassis tunnel, as described in kitcaranalysis_V2.doc (http://www.locost7.info/mirror/chassis.php)
I then found
a picture that shows how small the actual tubes are, they look mainly for show.
None of this changes how I'm doing things, I just thought you might be interested.

It looks brazed together? it thought that was illegal these days as it causes cracks

It'll be bronze welded (like every Caterham chassis ever made). Do a search it's been discussed half a dozen times.