OK - that probably justifies the delay.
The only thing I can think of that might save you is if the council messed up the process by which to transfer the liability to you.
They will have served a PCN to the hiring firm in the first instance. That firm then responds saying they aren't liable as you have signed a hiring agreement. The council cancels their PCN and sends a new one to you. Your PCN was deemed served on 18 March (2 business days after posting). If the notice of cancellation to the hiring firm was dated on 17 February or earlier, then they are out of time (they get another 28 days to serve the new PCN on you).
So, you could get in touch with your hiring firm and ask them to forward to you the notice of cancellation to see what date it was effective.
Not sure how responsive they will be, so keep an eye on the clock as I can't see any other way of avoiding paying this so you don't want to miss the discount.
Others may think of something though...