UKHeliBob:
I note that the screen is a shield and hence plugs into fixed pins on the Arduino. Are you sure that all of the pins used on the Mega correspond exactly to the pins on the Uno in their functionality ?
I have made an assumption that that is most likely the source of my issues as I have read that the Mega pins, although labelled the same, have different functionality in some areas. However, I am not skilled enough to know what the differences are or how to work around them.
I had thought the UNO shield would work on a Mega but maybe it just wont. Will have to see what others say.....