In yet another reply to myself- I carried out further tests as follows:
I saved a dxf file that I knew to be ok as
1) dxf R12
2) dxf 2000
When i loaded these files onto Windows viewers by way of a test I found:-
The dxf r12 loaded into "Brava" and "Edrawings"
The dxf 2000 loaded into Brava but NOT Edrawings"
My conclusion would be that the R12 format would be a more common
medium of exchange. This is backed up by a quote from another website:
"Although Autodesk keep updating the format, release 12 is a basic
standard that all CAD programs should be able to read."

It made me laugh-if you are a Windows user you have to pay for
Qcad-Linux you don't. Score one for Linux.