Ah well, that's a pity, nothing really jumping out at me there.
Mrs J has just gone out in Becky and, as she always parks nose in to our building, I always stand in the road behind her to be sure no traffic is coming round the bend as she reverses out. I was able to listen carefully to Becky as she started up and all of the tappets rattled lightly for about 3 or 4 seconds at the most before settling down to her usual pretty quiet tickover. If it was an oil supply problem I'd expect to hear exactly that and that it would go shortly after the oil warning light extinguished (when the oil reached the top end of the engine). So, not much help really, but I don't think it's what Becky does. I notice the sealing O rings all seem to be present and correct too. By the way, the front drilling on the cam cover goes nowhere as the engine doesn't need a VVT oil feed - which is why it looks a bit "gungy" because there's no oil traveling through it.
Back to square one I'm afraid. Sorry I haven't had any other ideas yet, but I'll get back on here if anything occurs to me. Have you tried the followers for slackness in their guides? Of course you'll need to have the cam lobe pointing up, away from the follower, to be able to do that. was just wondering if one, or more, was "sloppy" in the head?