If it is from the council and there genuinely was no penalty charge notice affixed to your windscreen, or you never received one in the post, write them a letter explaining this.
Enclose a cheque for �30, and say that you are quite prepared to pay the discounted sum, but that someone must have removed the charge from your windscreen before you returned to your car.
They should write back to let you know that they have accepted the discounted payment in full settlement of the matter.