I've worked out the answer to my question, it's to do with how the ILS display is presented: The display in the picture has a centre dot, surrouned by a circle, then four dots downwards and upwards, the line showing 'maximum safe deflection' is between the first and second dots on the lower scale, however, including the circle around the centre dot the bar is indeed half way between the centre dot and the bottom of the scale, ie half scale deflection.
Thanks for the replies, I passed the exam with 100%.