This has been discussed a number of times on Locost Builders. Aluminium isn't strong enough to make a space frame like the Haynes Roadster (in the same way you wouldn't make a roll bar from 2mm aluminium). Yes some exotic cars use aluminium in their construction, but its designed and developed from the start. Its not about stiffening up the corners or heat treating the welds, its just simply about it not being strong enough for this chassis design.
Built/IVA'd Roadster - RS2000 16v, ZX6R, Megajolt