Hey Peter, on behalf of the others and myself, thanks for getting involved with the discussion. I for one am running an iPhone 4S with iOS 5.0.1 on board and after reading your post, thought that I would wander out to the car where the Nokia CK-15W car kit was waiting patiently for my attention and give your suggestion a try. Home and sleep buttons pressed until the iPhone turns off automatically. Restart and...........no difference.
There is and always has been, a workable connection between the phone and the carkit but since the iOS 5 upgrade the car kit does not display the carriers name, signal strength or battery level on the car kit's display. Everything else is ok just not the display.
Bluetooth 4 is supposed to be backwards compatible but something has gone amiss with the coding and I cannot justify replacing the car kit (makes a very expensive iPhone 4S) with an "iPhone approved" version.
Did you have a look at the link provided by jkokmelk? It seems that we're not the only ones with BT and car kit issues.
Thanks again for the suggestion.