Editing tag fields creates entry\\entry\\entry etc.

This behaviour is limited to a few files. Attempting to change any field just adds to them, deleting the tag resets the entries to some default but won't let me edit them without appending \\entry. I realise that \\ is a useful and functional character but how do I stop this madness?

Usually, the \\ shows that you have several fields of the same type in a file.
Usually, MP3tag does not add more fields (of the same type) but either overwrites the contents or you have to add another field explicitly.

So I would suggest that you check the files for integrity with tools like foobar, mp3val and/or mp3diags.