Thanks for the tip Parksy, I'll give them a try.
Swift made their early (1980's chassis from aluminium. There was a significant problem with cracking when the chassis were 5 years + old. This IMO was down to "age hardening" and stress from "cold forming" the bend to form the A frame. Aluminium requires very skilled TIG welding to do any permanent fixing. Hence why bolting together is the best alternative. I used to work for the chassis builder for lotus cars and had first hand experience of their "glued" chassis construction. Have a look at
http://green.autoblog.com/2008/05/18/lotus-creates-lightweight-structures-division/ Lotus Lightweight Structures and you'll see some revolutionary chassis concepts.
http://www.lotuscars.com/engineering/en/competency-lightweight-architectures
As a good weld is far better than bolts that can rattle free, I was thinking of welding (my son is doing the welding) cross members in steel to stiffen the chassis to then carry the van body. I'm also considering making it dismountable, both for convieniance of reusing the flat bed trailer (13ft x 6ft deck) and also as added security as the body will contain our furniature and household goods.
We are trying to save the £900 per annum self storage cost. Plus I can't scale 13ft up a ladder anymore to get at stuff and my lad hates heights. It took me 30 mins to talk him down the ladder one day. He can just manage a set of steps. Must be very hard on him to have a dad who used to walk industrial steel frames without a worry in the world.