Would angle iron be worth using? If so, how might I need to treat it?
NO
Would an old circular saw blade from a chop saw be useful? Again, what would I need to do with the blank once I’ve cut it out?
Depends how its made, is it pure carbon steel, all the way through, or harder steel welded to a softer body?
I have a strip of drawn steel from B&Q. I’ve no idea how useful that might be but I’ll put it here because it’s available.
No
Right now I’m very tempted to use the blade from a set of secateurs but it just won’t span the swivel lock (water pipe fitting) by 0.8mm!!!!

I’m more than used to using a right handed knife. I believe it would be called a chisel grind here.
Stop being a tight bottom, and just spend a tenner.