Solved! The offending lifter came out of the rocker easily enough - using a knife edge to lever it out. All the oil channels/feeds were clear, in the shaft and rocker. The hydraulic lifter itself has a ball valve in the base, which admits oil. I probed this ball with the end of a paperclip, and initially it appeared very stuck - a little more pressure freed it up with a distinct 'click', although I had managed to dislodge it off-centre slightly; tapping the lifter sharply on a clean surface, oriented so the ball would shift into place, re-seated the ball OK. In order to minimise the time required for bleeding the lifter, I held the rocker with the hole for the lifter inverted and filled it with oil; with my thumb over the hole in the main rocker bearing surface to prevent oil escape, I pressed the lifter in and hopefully forced some oil into it. Bolted everything together again and everything is now nice and quiet - no tapping at all, and apparently no time required for bleeding the lifter as it was quiet straight from the start. I can only presume that the ball valve had become stuck by a build-up of varnish. Anyway, thanks for the advice, and general thanks for this forum - a great source of info which has saved me a lot of time and probably money.