Installed new spark plugs today (and new air filter and thermostat while I was at it). No improvement in idle.

Next thing to do is to clean the filter inside the fuel pressure regulator.

Cleaned fuel pressure regulator filter today. Here is mine after 247,000km. No idea if it's ever been cleaned before. I've only had the car for the last 54,000km.

(click on image for full size - apologies for the fuzzy image):
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A noticeable improvement in idle now. Not perfect but it's getting closer. I'll keep searching for things to check for.