Gotta love those undocumented features.
Yes indeed, #99802=0 does zero the Z axis DRO. Works for #9980n for n=0 - 5.
In addition, #99800=#99802 transfers the Z axis value into the X axis DRO. Full Read and Write.
And #5211 - #5216 are the G52/G92 offsets: touch them at your peril.
Thank you Stirling.
Cheers
Roger