So, in the hope it's just the hydraulic tappets running dry as the oil is not getting up to temperature and the right viscosity...... I started it up without the radiator cap on to stop the system pressurising. Ran it up to about 100 (normal for me is around 80) and let the fans kick in. Did that a few times, not letting it get too far over 100. The rattle subsided. Put the cap on and ran it 'normally' and the rattle eventually went altogether. Revved it a few times, sounded great. Cam covers almost too hot to touch. Also, as the fans went through 3 or 4 cycles the idle is much more stable. I seem to have a result. Compare my before and after videos.
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hedgerley's post in Worrying Rattle was marked as the answer