Wow you're spot on Matthew - I tried a test with exactly 10 characters in one of the Custom Fields and it worked fine.
This is good news as I can work towards making an import sheet that works, i.e. concatenate with spaces to add up to 10 characters.
Slightly disappointed for a couple of reasons
1. A lot of the fields are different with a mixture of text limits and character amounts in the data so this may become quite a bit of extra work developing the import sheet to be successful.
2. As you pointed out - this is a maximum value in the app not an exact value. There's no option to make the fields an exact value that fits exactly what I need. The import feature needs to be more robust with it's requirements for Custom Fields.