Okay, so sorry I've been a bit dead on the forum these last 24 hours but I have been scripting non stop to fix the lag issues that were occuring on some PCs.
Yesterday I finally got my hands on a PC suffering from the same issues. I have fixed it on that one and I hope that it will now filter down to the rest of you.
As a consequence we are seeing increased frame-rates across all our machines so this should improve the gameplay experience for everyone.