I changed my phone in the device section with no problem it worked for a little while but then it stopped working and whilst my new device is visible in devices it doesn't work and states I can't do anything till 1-3-2019"any ideas sky app also doesn't work
It sounds like you've run into the 'two registered devices and one change per month' requirement for the TV Go app. I have a Windows 10 tablet which I have used with the TV Go service and found that watching the content from the TV Go app was treated by Virgins service (with the above rule) as a different device to my watching it in the Microsoft Edge browser or indeed on the Internet Explorer browser all on the same physical device. My suggestion is to decide which one way is your preferred method for your new Sony Phone (personally I prefer the app) and then stick with just using that one way on your new phone. If you've hard reset (aka also known as a reset to factory defaults) and reinstalled the app on the same device this may also have counted as one of your registered devices or changes (even though its the same device).
When I had this happen to me I contacted VM Tech support and pleaded my case. They eventually reset my registered devices to start fresh and this got it working as expected. Stick to the one way of watching per device and this will help prevent a recurrence.