Be careful
Graham,
Be very careful, that rule you state is not correct, it may have worked for the example above but it is not a correct statement and it will get you in trouble with your understanding. The question below demonstrates this, using your method you would have got this wrong.
An aircraft is flying over the sea at FL 100, with a true altitude of 9000 feet; local QNH is 1003 hPa. What assumption, if any, can be made about the air mass in which the aircraft is flying?
When you carry out the working I described, True Altitude is less than Indicated Altitude on QNH, therefore the air colder than ISA.