OK, got that sorted using the relevant part of the customfunctions.php script from effervescence+
Is there anyway of incorporating the following into this script:
if (getFullWidth() === getFullHeight() OR getFullWidth() > getFullHeight()) {
printCustomSizedImage(getImageTitle(), null, 600, null, null, null, null, null, 'reflect');
}
else {
printCustomSizedImage(getImageTitle(), null, null , 400, null, null, null, null, 'reflect');
}