That would be interesting. I’m in Hampshire near Basingstoke.
Even if you aren’t local, if you could have a look at how the leaf spring is attached, bolts + number of washers and if it is slightly loose due to a gap between bar and plinth, that would be great. The PDF doesn’t highlight the leaf spring or the bolts in detail even though it does all the others!
I had an excellent chat with Sound-Hound (Rich) and we compared his S600’s with mine. Naim Support have also replied.
In summary, There should be a gap between the Leaf Spring and the plinths as per my pictures - There should be no contact between the plinth and spring. There needs to be a 4mm gap between the Plinth and Cabinet all round and no rubbing between plinth and cabinet.
As for the slightly variable movement, Naim highlighted that as they were hand built initially, and also the nature of wood, that there would likely be some change to them over time.
So all good. As Darkebear suggested, they do sound better after a couple of days to settle again following the maintenance. Well worth the effort to do this, not only as some of my plinth bolts were a little loose, but also piece of mind that all is well…
Thanks to all who helped and suggested things, especially @Darkebear, @sound-hound and Naim Support.