Thanks Binhex - BT were in fact hijacking the DNS.
I set the DNS in both unRaid and at the router to use Google and it worked a treat
One thing of note for any other users who have a similar issue; if you use a BT Hub 4 (like I have), then you can't override the DNS settings. You will need to either use a dynamic DNS site or use another router which will allow you to configure the DNS. You will also need to turn off all the BT Parental controls on your BT account to get this working. If you have a BT Hub 5, i believe you can override the DNS......
Luckily, I have a spare Netgear router so all working now.
Many thanks for the speedy responses