Sorry I don't know my local steel merchants. I looked on Google and the first on-line retailer I found sells 30x30mm mild steel box section at £8.30/meter, and the next sells 30x30mm stainless box at £18/meter.

Alu 30x30mm profile is £6.55/meter.

Looking a bit deeper I foun mild steel for £5/meter. Is mild steel suitable or should I be looking for 'engineering steel'?

Thanks for the advice on water-jet. A shame, as most precision engineers seem uninterested in small one off jobs. I worked shortly with a local one who did a bad job and our relationship went bad when I expected them to make it good. I've emailed a few online guys and they tend not to respond for small jobs.