Heard back from Corus, who have provided names of a couple of possible suppliers, (given that I wont be needing 20 tonnes of the stuff).
So we have:
Barrett Precision Tubes
Hub le Bas
edit: Have BS EN 10305 ERW in 6.1m lengths, prices range from £1.70/m to £3.15/m so a very rough guess would be £150-£200 for the chassis steel. Will need some extra for making up a jig to build it on, but these are good prices.
Both UK based, and from the stock lists of le Bas have 40x40x2 RHS in S355 grade, so emails sent to both to see what else they have and current prices.