Thank you for taking the time to reply! I enjoy these threads, not least because I read lots of older threads that are still full of helpful ideas.
I'm confident that the plinth is not flexing, however, the levelling feet that I have mounted do have flex, so that needs changing.. I've also added 50kg to the plinth to see if more weight helps, alas no.
The concrete slab sounds like an excellent idea, I can't find anywhere to purchase them so I'm left with two options: pour a concrete slab, or attach the lathe to the concrete floor. I'd rather avoid the latter due to concerns over the bolts cracking the concrete due to vibration (unfounded?) but how on earth do you level a concrete slab?
*EDIT : as long as the floor isn't hideously lumpy, would a rubber sheet be enough to ensure good slab to the floor contact?