Then it got worse.
The route-sheet I'd planned to get from the ferry port to the town sent us directly away from the town, down a dead end. We followed our nose into the centre of Dunkirk, and tried to pick up the route out. We had several false-starts before finally breaking free on the lovely smooth canal towpath which was to lead us to Belgium. There was a BANG, and the canal path was no longer smooth and our rear tyre was no longer inflated. The tyre had a snakebite puncture, the inner tube had at least twelve holes (I gave up counting at that point), and the rear wheel required percussive maintenance to make it round enough to hold a tyre.
The tyre was booted with cardboard, the old inner discarded, and we limped cautiously on for another ten or so km, keeping our eyes peeled for a bike shop. After Bergues we flagged down a pair of roadies and got directions to the nearest bike shop, about 15km away (and in slightly the wrong direction) in Wormhout. Once in Wormhout we found that we'd forgotten to bring the helpful cheat-sheet of parts-of-a-bike in French, so had to do a lot of mime and pointing to explain what we needed (the bottom bracket was also loose, so we had to borrow some tools). The new tyre was fitted, and promptly deflated while we ate lunch, because we'd pinched the inner tube when fitting it. The puncture was fixed, and it was three o'clock in the afternoon and we still had 80 km to do that day.
Eventually we got to Roubaix, and promptly became very lost. Getting in to a town that you don't have a detailed map of is easy: getting out is far harder. I think we must have cycled around three sides of the town (including some pretty nasty dual carriageways) before eventually picking up the road we wanted. It was getting late, we were tired, and looking forward to a campsite and a shower.
We rolled in to the campsite (as listed on the tourist board website), only to find a derelict set of buildings and a highly amused Frenchman who informed us that the campsite had closed ages ago. At this point, dusk was beginning to fall.
With not a lot else to do, we continued on our route, looking for somewhere quiet and discrete to pitch a tent. Eventually we found a welcoming canal towpath which was wide enough for a tent, crawled into our sleeping bags, and passed out with 130km under our belt on a very stressful day.