To answer your questions:
The line has worked ever since I received the adaptor from VM to put in the back of the hub.
The area fault seems to be resolved as there's nothing on the status check and a bog-standard £5 ordinary phone I'm using at the moment shows the the phone line since switchover is working.
The technician is also test the line with their test phone and it worked fine.
If the line works with a simple handset then you need to look elsewhere for the fault.
The BT Premium phone does get lots of mentions on here with problems. There is a 'nuclear' reset option here at message #17.
BUT, if you do that, it will wipe your phone back to factory settings so you will lose your contacts, call screening settings and other phone configuration. There seems to be a Bluetooth option though to re-sync contacts from mobile. So, be aware of all that lot if you try it and use at your own risk.
Also make sure the satellite handset batteries are fully charged when testing.
On the page footer of the manual
there is a freephone 0800 number for product support for that model. You could see if they have any wisdom to share on why the phone isn't working.