Discovered on the ferry across that the route from Dieppe to Paris goes through Rouen, which like many large French cities now has a Low Emission Zone. Even sticking to the main road takes you through the zone - so you need to have a Crit Air sticker. Apparently it’s now ANPR enforced.

I applied for one on the ferry - very easy if you have a scan of your V5. The official site is https://www.certificat-air.gouv.fr/certificat and you can switch to English via button at top right. After paying €4.91, they will post a sticker to your UK home address. Within 3 hours of ordering, I had the official invoice through by email, which also included a PDF copy of the sticker - apparently you can print this out and leave it on the dashboard if you park in a LEZ, if you don’t get your sticker in time.

So it is possible to stay compliant, even if you’ve left it too late to get a sticker (which lasts the lifetime of the vehicle BTW) 2019 L405 Vogue SE 4.4 SDV8, Byron Blue