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...