Scott M wrote:
Just to add my 2p, I'd be expecting the silver caps to unscrew at the
gold section. (The top is then held to the cartridge by a grubscrew.)

This had occurred to me, but nothing happened when I gave it a quick
try. Do you think the join is such so the cap is the silver bit, or
does the cap have any of the gold rings too?

I can see what looks like a join where silver meets gold, and can get
the blade of a knife to sit in the groove, but working it round doesn't
widen the groove, just removes some limescale, so perhaps you're right
and it unscrews rather than pops off. I'll purloin some Marigolds and
see if the extra grip helps.

Thanks, Jack.