Great replies and help - thanks pruners
I'm comfortable with the concept of the column of air expanding so in flight, flying from an area of lower temp to higher temp will cause the altimeter to give a lower reading than your actual altitude i.e. your plane will be HIGHER than the indicated altitude.
However, there seems to be some disagreement on temperature and pressure when on the ground. If I'm on the ground (say airfield elevation is 1000ft) and I set my altimeter for airfield elevation. An hour later, the temperature has increased by 25 degrees (I have not changed the altimeter setting) - will my altimeter read higher or lower than before?? My assumption was that it would read higher, temp increased, air pressure decreased, higher reading.
Once again - appreciate the wealth of knowledge and replies........