as it happens, that's exactly what I've done.
protected object grid_OnExternalDataBound(object sender, string sourceName, object parameter)
DateTime dt = (DateTime)parameter;
return dt.ToString("dd/MM/yyyy HH:MM");
Although I'm confused why I've had to take this extra (and seemingly unnecessary) step. None of the other module grids I've looked at in the project seem to bother with this.. the dates just display correctly on those