Well, you kind of pointed me in the right direction but not in the way anyone intended.
 
I exited sonar and ran the stand alone version.
The GUI opened up along with a message saying something along the lines of "This synth is not authorised yet, please visits the Command Center tom authorise it. Until you do, the sounds will created random blocks of silence"
 
But I'd only downloaded this version earlier, all authorised through the CCC.
 
So I reinstalled it again and now it seems fine.
 
I'll mark it as solved but I don't know why
 
Thanks for your contributions, Steve & Paul

 
CCC is doing some strange things as of Manchester.