We're puttin it on the next Demon Barbers album, can I change a few words (which I've probably done already) and claim all the prs and mcps for it?!

As long as you put it down correctly as "Trad. Arr." by you, yes. In fact there's every good reason to: if you don't, somebody else who doesn't deserve it will get it from another copyrighted arrangement, or it will just go into the big pot that feeds the like of Paul McCartney & Elton John from the "unassigned" stuff. Best idea is change the title too.

As I understood the famous fallout of singing men between M. Carthy & P. Simon over Scarborough Fair when the latter nicked the former's arrangement and it appeared on record as by Simon, MC would have let it go if PS had credited it as "Trad. Arr. P. Simon". That's the modern-fangled folk process. We may tut-tut at it, but since Mr. Anon isn't around to get his royalty cheques, better a struggling folkie working to forward the music than fatcat pop songwriters who had nothing to do with it at all.