It's a bit late now, as you seem to have admitted that the ticket was 'not displayed', however, I have got off a couple of fines by claiming that the ticket was 'clearly displayed', but that your attendant 'seems to have ignored it', copy enclosed etc. etc.
This seems to work when the Terms & Conditions do not explicitly say that the ticket must be displayed in the windscreen, as then you can say you stuck it on a side window etc.
I have even gone scratting round the car park looking for a discarded ticket that covers the period that I was parked and received the ticket!!