So I took the Z axis plate off tonight and swapped the v-wheels to some other spare (new) ones I found. The originals were not loose, only a tiny bit of play and no obvious wear or damage. The new ones were much better, and tensioned correctly - no play. Odd and a little disappointing as the machine is not that old and hasn't had that much use. These were solid Extreme V Wheels for reference. I've ordered a set of steel v-wheels to see how they fair, for the 8x Gantry plate v-wheels. May change more if these work out. >How deep is your initial plunge? @Giarc - not that deep - usually 2-3mm and less than 1/2 diameter of the bit. Again - many thanks all for your help!
You'll notice OpenBuilds doesn't sell those - they wear the rails - not ideal Proper Xtreme wheels shouldn't wear abnormally unless the eccentrics are set overly tight.
I replaced the wheels with polycarbonate wheels (Extreme vWheels) and issue is sorted. Thanks to all for your help.