Thanks for all the advice. I was concerned about grease collecting dust, these screws are quite exposed, and WD40 is always something of a mixed blessing in many applications it would seem. The chain bar oil seemed reasonable to use as it is designed not to fly off moving parts. It's also cheaply available at most hardware shops in Australia.

The ballnuts are drilled for grease nipples but don't come with them. Should I really be fitting nipples and buying a grease gun? Will new grease applied in this way push out any dust that gets in? I was rather put off by my experience with the aerosol Lithium grease but don't have the experience to know what's best.

Yesterday I found some slight backlash in one of the long axis ballnuts so it looks like I've been too lax with the lubrication recently and have already done some damage. I'm assuming (hoping) the screws themselves are OK and have asked Fred at BST Automation for a quote for 2 new ballnuts. I will have to do a better job of looking after them in future!

Thanks again

Kit