In reply to Andrew Cox:
I looked around and found the format defined in the PKM loader and at the top of it's source file in Ericsson's texture compression tool which is here: https://labs.ericsson.com/research-topics/media-coding/ericsson-texture-compression-tool-v2-74?download_terms_js=1&download_terms_file=system/storage/serve/524/.
I can send the relevant bit directly if you (future reader of this page) contact me.
FYI, the source is no longer available at the previously linked location. It is now on GitHub at: https://github.com/Ericsson/ETCPACK