Hello Shahpor
Thanks again for your additional comments.
There have been no modifications to the system since I have had the car.
I have enjoyed having the full screen for a couple of years since I purchased the car, and then a few months ago, for no apparent reason, it reverted to the split screen! I had not made any adjustments or alterations to any settings - it happned just as I was driving!
Prior to this, the split screen only appeared if I used the radio, etc., when the spit screen would appear, disappearing automatically when these functions were closed.
Michael