File created after render only contains few 100mb

I'm using 17.2 on a PC with plenty of power.
Footage is shot at 25fps although the settings in DR are 24 - this has never caused any issues, especially on rendering. All finished projects have rendered out at UHD without issue.
Final timelines, usually 15-25 minutes, have all rendered successfully until today!
Come to render and the finished MP4 file contains a few bytes of info - DR says it has successfully rendered !
I'm pulling my hair out (if I had any!) to work out what has happened and what might be causing it.
Have tried resetting the project settings, but as I didn't really change much, if anything in the first place, it has made no difference.
Any ideas?

[Peter Chamberlain]

You should shoot media at ur delivery frame rate, and set the timeline rate to this same fps prior to placing clips in the timeline

Try, start a new timeline but in advanced settings set the frame rate to 25, then copy and past the bad timeline into the new, clean up the edit point and then render.

https://www.blackmagicdesign.com/produc ... e/training