We sit in the sun, quietly digesting, trying not to think of the bivalves splashing about inside, and watch the bustle on the beach below, which is less of a place for sunbathing and sandcastles, and more a giant oyster factory – long, low racks stretch right down to the low tide mark, laden with huge wire bags of bivalves. Workers in waders rolled down to their waists in the sunshine wander among them, turning the odd bag and loading a few that have clearly passed some mysterious test onto the tractors that chug in a steady train down the slipway behind us – one flops off onto the road with a great jolt, and a chino-clad tourist runs to the driver’s aid. His spectacular failure to even get the bag off the ground, and the nonchalance with which she picks it up and tosses it back onto the trailer, have us cheering like a seaside Punch and Judy show.

      In fact, all this activity proves sufficiently fascinating to persuade Matt to cycle back to check on the museum before we head on to Saint-Malo – at which point it begins to rain, and not just rain, but pour. Shoving our bikes hastily behind an abandoned boat, we rush in the direction of the entrance, only to find it still locked up. When I eventually locate a living being in a shed nearby, she tells me, and a vexed-looking French couple who have followed me in, that they’re not actually opening for another 30 minutes. The French observe this is not what’s printed on the board outside. She happily agrees it isn’t.

      The terror of missing out on the opportunity to get better acquainted with the many creatures currently dying a horrible death in my stomach must show on my face, because Madame suddenly relents and offers to let us into the museum early instead. I’m delighted: not only will this offer entertainment (though, given that it turns out to be largely devoted to a collection of seashells from around the globe, it doesn’t actually provide very much of that), but it’s warm and dry, too. Perfect.

Once we’ve exhausted the gruesomely detailed biological diagrams of the oyster and mussel, both of which make me heartily regret my dietary choices over the last 24 hours, we stand and watch the rain until a sufficiently large number of pensioners in sensible waterproof clothing have gathered to make a tour. This begins with a lengthy video in French, during which I think I learn that the bay of Mont-Saint-Michel produces about 15,000 tonnes of oysters a year. Their superior quality is attributed to the dramatic tides, which work the muscle holding the shells shut, rather like one of those electric six-pack machines advertised on daytime television: ‘Strong muscles are important,’ the narrator explains ‘as they might be in transit for up to three days if they’re destined for Japanese tables.’ Things could have turned out worse for my 15-year-old, all things considered.

      I read on the way out that oysters are ‘your best friends if you are slimming … the queen of all diets, with only 70 calories per 100g’. So French.

      Outside, the rain losing heart now, we squelch down to the shoreline to peer at the racks in the distance, where bags of tiny oysters grow into adults, helped by the farmers, who regularly turn and move them to more spacious accommodation to encourage expansion. Once they’re big enough for sale, the oysters are brought up to spend a week or so in an oxygenated, temperature-controlled tank (5–8°C is apparently optimal) to ensure they’re in tip-top health before they’re sorted, with workers weighing each shell in their hands to check they’re heavy, and thus full of water – any suspiciously light ones are discarded as probably dead.

      ‘In the run-up to Christmas, we employ 69 people here, and they grade 1,800 an hour,’ our guide explains, ‘so they don’t have much time to chat.’ Matt shoots me a meaningful look as the party moves on to the climax of the tour: the dégustation, the oysters already laid out on long tables for our gustatory pleasure. Frankly, I’m relieved to make our excuses and hurry off, though Madame is shocked that we’ll miss the best bit. ‘Ah, les fous Anglais,’ I imagine her muttering as we hastily exit through the gift shop, but I’m content to live up to the national stereotype if it means I don’t have to look another oyster in the frilly gills for a day or so.

Though his check-in opens in half an hour, and we’re still 15km away from the port, Matt confidently assures me as we unlock our sodden steeds that his ship doesn’t actually sail for hours, so until Google Maps sends us down the side of a farmer’s field with mud as deep as our pedals, we make a fairly leisurely pace west.

      Fortunately, once we divert on to the main road my phone is so keen to avoid, it’s a fast run into Saint-Malo, although the town sprawls wider than I remember, and the ferry terminal sits bleakly, in the manner of such places, on a dual carriageway with nowhere for a farewell drink but a warehouse advertising cheap crates of beer to British booze cruisers. For the last time, I give thanks for my companion’s unerring nose for ‘just a quick one’, which leads us up the hill to the distinctly un-Gallic Cunningham’s Bar from where we can enjoy the sight of Matt’s boat patiently waiting for him as we raise a cider to my continuing adventures.

      When the funnel begins to smoke, I reluctantly suggest we should probably make a move and insist on chaperoning him all the way to the ticket gates, more for my benefit than his. As he disappears cheerily through them on his way to that much-anticipated buffet, I feel cast adrift like a tiny oyster larvae floating free in the bay. Here I am, on my own, in a strange place, not knowing where I’m going to stay or eat tonight, and a whole month stretching terrifyingly ahead of me. Well, nearly alone. I still have that pied de cheval to keep me company.