It worked like a charm. I used it once, and my dandruff went away. I used it twice more, but there didn't seem to be any point in continuing with it as there was no dandruff to get rid of.
I've kept that bottle for 26 years, in case the dandruff came back, but it never did. Finally, though, I've decided that it's probably gone off by now, so I'm chucking it out.
A shame, there were still six shots of it left.
A quick poke around reveals that Dannex still make anti-dandruff treatment ("suitable for vegetarians"), although I've never seen their products anywhere except my barber's back in Yorkshire. It looks as if they've changed the active ingredients, though, to Cetrimonium Bromide, Imidazolidinyl Urea and Sodium Methyl Paraben; previously, it contained only Cetrimide B.P..
Bizarrely, it seems that all these years I had this original formulation of Dannex I could have used it as a mouthwash, too.