I emailed DFDS re parking at Dunkirk yesterday, they responded that they had no facilities for parking after disembarking from the ferry, and the emails content left me with the impression that they didn't wish to accept any responsibilities for parking at the terminal. This response came from the Dover office, so I assume it is their standard policy, which I guess is understandable in these times.
It looks like we will just have to see when we arrive what the 'real' situation is, I will of course post a comment, so that others can benefit from our situation.
We don't travel for a couple of weeks yet, so perhaps there will be further updates. Many thanks for your input Tim, as is always in these matters, the latest information with a date, is the best advice.