Great find john973, and thank you for your posting and the tip within it, many times in the past 18months or so, here on the forum people have ask how you can stop the radio coming on when starting the car and you appear to found an answer, John so well done and thank you.
I have just tried it on my MG5 and it works.
The only thing to be aware off is when you switch the radio back on the volume is where you left it so when turning it off as John has Described above, turn the volume down before you turn it off job done.
Les