The council may accept late representations, so submit something to them now, and see if they respond.
As it was a hire vehicle, the hire company get the first PCN, and then can submit reps that they are a hire company and give your name and address. The council then cancel the first PCN and serve a second to the name and address they have been given. 28 days are allowed for service of each PCN, so I am not surprised about the delay in you getting a PCN.
They may be about to serve a Charge Certificate which adds 50% onto the PCN penalty. If the Reading website allows you to look at your PCN record, see what stage it is shown at.
Your case shows the importance of addresses recorded at hire companies.