Thanks for your suggestions.
The script is error free. If I delete temporary internet files the new version of the script file is downloaded and works ok. It can't be the dev environment because if I edit the script file within CRM, in the web resource dialog text editor, add an alert,
save and publish, I don't see the alert until I delete temp internet files.
To avoid having to delete temp internet files I've started using Fiddler's Auto Responder utility which allows me to redirect to the local copy of the script files and this works fine. It's also quicker than updating CRM and publishing.
I'm still curious as to why it's happening though.