Hmm now that I think about it I think it was actually Nulon lifter free that I used, not worn engine treatment. It didn't seem to make much difference, and the instructions say it needs to be used every oil change, which makes me think it just thickens the oil, or something similarly non permanent. The strange thing was that with the old oil the noise was fairly subtle, I could only hear the ticking when going past a kerb or a fence and the sound reflected back. After the oil change, the racket yesterday for a couple of seconds was certainly audible.
Note the clatter at startup was actually once I'd changed AWAY from Magnatec. It's only with the thicker oil that it's making the more horrible noise, which is why I'm thinking I've probably gone in the wrong direction with regards to viscosity. So what I need is a high kilometre oil that has the extra protective additives, but is NOT thicker to compensate for leaks. All of the high kilometre oil products I've seen are thicker...
BTW: the service was neglected but the engine certainly wasn't thrashed during that time, in fact I'm probably a little too laid back, I doubt it often goes over 3000RPM.
I gave it a minor italian tuneup yesterday on a quiet 80km/hr road but even then I only got up to 4500RPM in first.