I've contacted the Live Chat Help people as I am still having trouble uploading subscribers and they advised me to try through an Incognito Page as Chrome runs some browser extensions that can affect things. It worked, so has solved the problem for today. They sent me a link to disable browser extensions so I'll try that and keep my fingers crossed.
https://support.squarespace.com/hc/en-us/articles/206545197-Disabling-browser-extensions