Hiya,
Had the same problem with my JAR PPL (although it is not a commercial licence, I know)...
The CFI at the club where I wanted to rent an aircraft was saying I could only fly with an instructor...=Total nonsense....!
After a long debate, after arguing that in the privileges section of the licence, it says that you are entitled to fly any aircraft registered in a JAA country, we eventually phoned the regiona district of the DGAC and the woman on the phone said it was OK for me to fly solo with a UK issued JAR PPL....
If I had not insisted, I would have been stuck with an instructor (who was a nice and competent chap, anyway!)
Cheers