Thanks for the replies. Here's some more information:

This is a 645 frame and using a metal reel. Is it possible that the reel was wet when I loaded it and the emulsion stuck to it? I've loaded a lot of rolls of film wrong but this one felt and looked OK when it came out of the tank. And I suppose if that was the case then the film edges would look bad too and they don't. Film edges look good and the film ID printing is clear (Delta 400), it's just cut off in by the film holder in the scanner. As you can see the exposure data developed just fine.

Definitely not anything in front of the lens as there are 5 others (all from one end of the roll) from a few different times and places.

The negatives are a little bit 'milky' you could really see it when I made a contact print from this roll. I know that my fixer is on it's last legs could this be a fixer issue?