I did have a suitable O-ring, fitted it and tried a test re-assembly without
lubricant. The result was a spectacular leak from around the O-ring. So I
obtained some silicone grease (exorbitantly priced from B&Q - Toolstation
denied all knowledge of the stuff) and tried again. The "grease" proved to
be rather more liquid than solid and was absolutely useless: it simply ran
off.

I gave up temporarily but later took the tap spout to a small plumber's
merchant (where I should have thought of going in the first place). There I
was told that there should be no O-ring: the sliding surfaces simply rotate
against each other. What about lubrication? Oh - use Vaseline, the chap
said.

Bert