Thank you one and all.
Using UG NX 5.0.6.3 and UG/Post Builder 5.0.3
In the UG/Post Builder 5.0.3 library, I found a siemens post, though it is not specific to a Sinumeric 840D.
Have created a basic 3 axis mill post in mm and have got some output G code, which looks OK for starters.
Will need to fine tune, for CNC Operator information headers at beginning of code.
RdMcC
Dell Precision PWS380, Pentium(R) D CPU 3.2 GHz 3.5GB of RAM, NVidia Quadro FX3500 with 256MB of RAM, Win XP SP3 OS32 bit, UG NX 5.0.6.3, UG/PostBuilder ver 5.0.3