Hi Angus:
I also experienced a crash when trying to open the file. This is why it is important to have incremental back-ups. Any file can become corrupt over time. If you don't have a back-up, you can work through this document:
https://helpx.adobe.com/indesign/kb/troubleshoot-file-issues.html
Note the final section has an email address to send the file for review: "If no resolution works, share the affected file with us at idfile@adobe.com. We'll investigate the issue and let you know if the recovery is possible." I believe Adobe USA is closed for the week, so you might not hear back for a week or so.
There is also a commercial service that may be able to recover the file. https://markzware.com/products/file-recovery-service/
Once you regain access to the file, please be sure to make regular backups. Now you can see why it is so important.
~Barb