I forgot about the Celtic Folkweave version. It's really nice. Strangely enough though, in that version the lyric seems to start in the middle of the fight with no lead-up, which is unusual. So that wasn't a source of clues for the missing lyric unfortunately.

"Set you forth for to fight the nine" sounds plausible as a lyric, but I'm definitely hearing an 'l' in the first word, as if it's "selt" or something. If asked to write what I hear phonetically, I'd probably say "sell teal forth" or "sail teal forth" - there's definitely an 'l' and a 't' in there. I can't work out what actual phrase would sound like that though. There is, of course, always the possibility that Mick confused or fluffed the line (maybe it was supposed to be "send him forth" or something) - I've come across that in lyrics before.

Sally is also plausible as a lyric in the context, but I'm just not hearing an "alley" sound in there. Oh well I guess I'll either sing what I phonetically hear or just substitute another version of the line. If only the lyric was as straightforward to work out as the guitar part!