Now refitted and it works! The range of delays isn't quite right, though, variable from almost none to maybe 4sec. I could fit a thicker cap of some sort to increase the plunger stroke, by making it stay in contact with the plate on the linkage for longer, but I suspect the 48-year-old vacuum line and white knob might not be entirely leakproof. The delay capsule itself seems vacuum-tight if I push the plunger in and press a finger over the pipe stub. The plunger stays in position as long as the finger stays there.
Anyway, everything on my Rover now works – the last fix was the clock, again with this forum's help, which had done the usual fusible-link melt. So it's a milestone day.