As the title suggests I've created a mix of a project by bouncing to a new track. it is complete from the beginning of time.
That is the clip is at 1:01:0000
I export it as an mp3 to send for others to add tracks.
The tracks I get back seem a little off so I re import my own track and find the clip begins where it should but the actual audio is offset by about  48 ms.
 
any ideas?

Try "select none" before export...
I think select all highlights the clips but if the first sound is not on "O" then the export is taken a little later.
There should never be sound on "0" anyhow.. 

All mp3 encoders add a little silence at the start of the file.

brundlefly
All mp3 encoders add a little silence at the start of the file.

Wow, that makes sense. Makes sense as to why it's happening.  It's really useful to know these things.
 
Glad I now know. Thanks

Yes, one of those little gotchas. Good that you caught it. It's best to share .WAV files when collaborating for this reason. If you zip them up, you'll save a little space without losing quality.