I want to change the radio unit on my s type which has 2 small screens to a large screened unit but the loom is different , is there an adapter I can purchase .

 

If you mean you are changing your non premium head unit to an alpine premium nav capable unit ,
Then this is a major job indeed ! It has been done , but needs a lot more than just the screen . You would be better off to go on eBay and buy the double din screen update unit with Bluetooth and Nav like this or similar
http://www.ebay.com/itm/7-Double-2-D...sAAOSwxvxW74sL

and a double din dash facier surround kit !
There is also a conversion plug and loom to save cuting the original plug aswell but that won't help with the alpine premium upgade . As you would still need to adapt the screen plug!