OK thats sods law. Just got a reply from PayPal (24 hours later) It turns out that you need to disable 'Encrypted website payments' in your PayPal account settings under Settings -> My Selling Preferences -> Website preferences I don't know what happened, when this got turned on but after turning this off everything works again. I would like to also point out that all of my site and services are encrypted with a valid SSL Certificate (I just double checked everything) and I don't remember turning this on so I'm guessing that its been on since day dot or has been added by PayPal as a security thing that I missed.  
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