Well I've started cutting and managed to aquire a sheet of 24mm ply (bloody heavy).
I am thinkng about building the base upside down and tacking it all together.

First I will draw out the base frame and fit blocks to all the edges ( got a bag of brown plastic block joint things and 25*25 Alloy angle) screwed to the base to hold all tubes in place.
Then spot weld all tubes together ( all clamped in place first ).
I will then turn it over and if all's well it should lie in between all the blocks (being symetrical). This will let me know if all is square and flat.
I will then clamp everthing in place and solid weld all the top faces of the tubes.
At this point I intend to take a router and make small channels in the build board so that I can turn the base over and reclamp it and solid weld the rest of the base joints. I can then build the rest of the chassis by tacking everthing else in place on what i hope is a near perfect base.
When welding the joints solid should I weld from one side then the other or should I weld from front left to rear right in order to minimise distortion.
Sorry about the long post lads hope you think i'm going about it the right way if not TELL ME NOW as i won't be starting fora week to ten days as with a wife, and two daughters that have just bought a flat my time is well in demand.

whilst i believe you are very much on the right tack with the plywood and clamps, im not sure its ideal to fully weld the base before you have it triangulated. I would tack the full base, flip it to make sure its symetrical and flat, then add all the verticals, top rails and diagonals, and then fully weld it. This should, IMHO, minimise the distortion

Sounds good to me, the only thing I'd suggest is to grind back the welds rather than route the base board, they're going to have to go when you fit the floor aren't they?

Mick