@hillary I did find the root cause, and it turned out not to be LFS
For those of you who have had LFS added toa repo, and have then removed it, but are seeing this issue, it is a good idea to follow the steps here, as this was part of the issue.
However the solution for me was to change a line in my .gitattributes file from * text eol=crlf to * text=auto eol=crlf.
This line had been added to make sure myself and the other developer on these projects pushed code into the repo with the correct line endings, but I noticed it was also changing image files, which would then fail on build. Adding text=auto tells it to only change line endings in text or binary files and not in images, I had to recommit the image, but this fixed the issue.
Thanks for all of your help and advice.