Tried a simple dome.
That gave the same error window on my sim version, but worked ok in the workshop.
So I saved the gcode onto a stick, and brought it back to the house (warm !) and opened it up in sim axis ok.
Rather than spend time trying to get the sim to work by finding out what the difference is, I'll use this as a method for now.
If anyone could say what I need to do to rectify the problem, I'd still be very grateful.
John
