Delighted to report all is now well. When 'old' HLC's removed there were 5 that were appreciably shorter than the other 11 - presumably hadn't retained oil?
Having installed the full set of non-MB replacements the engine is wonderfully quiet, cold or warm.
The lesson here seems to be that a surprisingly large proportion of genuine new parts can be faulty.
Interestingly, the TRW components were identical to the MB ones - even to the same part no. stamped into them.
Thank you again for your prompt advice.