I initially received a confirmation email before the season began. I thought that was going to be the PDF I needed for the whole season; it confirmed it was a season ticket in what car park.
However, it now appears that you do have to log on prior to every and (although all show on your account as a PDF link so you prefer you can print every one off altogether) and it also shows your reg.
I agree with the above, it is a bit of a pain if you car share (as I do too) but seems pretty seamless to end you reg now they've sorted the website (now sure if it changed for Car Park C just because I highlighted the issue of originally not being able to change it online, but it's working fine for me now).
If you do have problems, I'd recommend emailing them directly. They are very prompt at getting back to you (well I experienced a prompt response any how)