Thanks everyone for your feedback, it's been immensely helpful!!!
So, despite the rather ordinary weather here, I did indeed spend a fair chunk of today under the car. Since I was there and it was also due, I did an oil flush, filter and oil change. Figured I might as well.
Sean82, I was reading your thread with interest, since it sounded almost exactly like my issue! Mine was not quite as severe, but very similar.
Anyway, after removing the gods forsaken plastic shield, I drained the oil, removed the filter and all that nice stuff that I'm familiar with. Afterward, I looked for the parts described by BigBadBenny and sure enough there they were, with the blue plugs as described. I'd spotted the ones on the top for the intake but I hadn't really noticed the exhaust ones before. Very pleased to note they were a lot easier to access and remove than I had initially feared!
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Removal was very straight forward, gaskets were in excellent shape so I didn't feel it necessary to replace them with new ones, though I'll keep an eye on them just in case I'm wrong. It took me quite a while to spot the filter, initially I though they'd already been removed! When I finally did see it, it took a bit of coaxing with a small screwdriver to convince them it would be best to leave. I do wonder if they're been replaced at some point since they were completely intact, not brittle at all. There was a little debris inside but not much, though the filter mesh is extremely fine.
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The drivers side one was a little more difficult to get at, since the exhaust gets in the way, but I managed to get that one off too. It was in the same state, healthy looking gasket, filter in place, no debris at all this time though. Pulled both of them apart, gave them a thorough cleaning, removed both filters then put everything else back together. Reset the ECU too.
After filling the oil I took it for a test drive/log. Results?
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To me anyway (and I'm very new to this so feel free to correct me if I'm wrong) this looks much better! Timing for the VVT now stays within a degree or two for each side, so I'm thinking it's nailed.
BUT! Still getting the occasional erratic idle! Going to keep looking through the log file, clearly I still have a lot to learn about this stuff.