In answer to my own post...
I found 20mm diameter 1.5mm wall thickness tubing available in 6m lengths from Goodman Steel in Reading. They were very helpful and even threw in some steel plate (for gusseting & mounting lugs) on the cheap.
On advice from an engineer friend, I bought cold drawn steel (CDS), with medium carbon content (AKA mild steel). The CDS process apparently aligns the grains along the length of the tube, increasing strength - particularly in bends. I chose mild steel as it is easily welded - increasing my chances of doing a decent job welding it together. Also mild steel requires only basic welding skills and equipment, meaning if it breaks it can be repaired easily.
It took me ages to find someone who would supply in relatively small quantities. I ordered two 'lengths' of 6.1m at a cost of £75 (or £6.25 per metre). Hopefully this will be enough to allow me to experiment and make mistakes.
My welding skills are progressing very well. I'm trying to practice for a few hours a week and have made myself quite useful building bits & pieces out of scrap metal sourced from my local tip. It's enormously satisfying and although I don't think I'll save money making racks and boxes myself, the experience is worth it. I'm confident that I will be able to do a decent job.
Next project is a tube bender - I'll post the results here.
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