Yes I know I could have bought a flag for not much money, in a variety of suitable sizes. But where would be the fun in that when I could spend HOURS of my life instead making one... Duh, what a pain. Cue cheapo heat transfer paper, an inkjet printer, some rip-stop nylon and an iron. After much failed experimenting it turns out:

1. Get the iron as hot as you can without melting the nylon (three dots on mine)
2. Use a piece of something hard underneath e.g. MDF board, not an ironing board
3. Press hard while heating for 30 secs or so
4. With help from a brave assistant, peel the backing paper off as you run the iron over it so it's as hot as possible. Ignore the instructions about letting it cool first - doesn't work.

Yeah, got there in the end...