Hi Stuart,
The .FIT file protocol is available to ANT+ members. Some of the content within the file will be ANT+ interoperable and some will be proprietary. This is analogous to the paging schemes that we use in the broadcast profiles where a page space is reserved so that devices can broadcast messages define by the manufacturers.
The File protocol description document is being reviewed for release now. I can email you a preliminary version. The software development kit for activity files is not complete yet.