Well,.needless to say, I got lost... But at least I didn't end up on the Autoroute.
I took the correct gate (as advised by a lovely young lady who was looking out for the single cyclist on the boat), and found the Route Des Dunes OK, but I didn't see the motocross place and thus missed the gravel track. So, I ended up going through Mardyck to get to Loon Plage, not too far out of the way really.
In my defence, it was dark, and I'd only had two hours of fitful sleep. That made for an interesting ride to Bergues before sunrise...
I've still only had that two hours, too.
I'm in the very sunny Grote Markt in Veurne now,.trying to think of a compelling reason why I should go to Diksmuide before I go to Ypres. I might get a small beer and ponder that conundrum.
Oh, and to the OP; nothing to worry about, follow the instructions and you'll be fine. Bergues is a nice little town, too.