I have season ticket, displayed in car. Slipped a bit, but still clearly visible. Got parking ticket, 90 quid, 45 if paid within 15 days. Emailed them, got reply that they "hope to be able to reply to my query within three working days, but it could take up to fifteen" - so if I wait until then and they don't believe me, I end up paying 90 quid. Oh and they "cannot resolve these queries by phone, only be email or letter."
You can get a weekly e-ticket, where ticket inspector punches car registration into computer to show that you have paid, but this apparently doesn't work for a season ticket.