The last morning at Kunzmann’s. We had done most of the packing the night before, so there was nothing to do but go down to breakfast at eight and eat slowly, which is a luxury I do not often allow myself on the final day of anything. The dining room had the settled quiet of a place where most of the guests have already left or are about to. Dry light at the windows; the promise of a clear run.

The useful thing happened over the rolls and coffee, which is generally where useful things happen. A man at a nearby table turned out to be staying at the hotel and working a stall at the show, one of the stalls Ochi and I had walked past without stopping. He was with Separ, who make filters for diesel, the marine and industrial and automotive sort that strip particles and water out of fuel before it reaches the engine. Water in the tank is the kind of small, stupid hazard that can finish an engine in a place where there is no help for a thousand miles, and so the conversation took hold at once. His stall had been linked to Bimobil, whom we had deliberately not sought out, since Bimobil build whole vehicles and we are building our own. Filters, though, are exactly our concern. We swapped details. He offered to talk it through properly once we were home, and I mean to take him up on it.

What he said dovetailed with a plan for the Sprinter that I have been turning over for months. The idea, borrowed straight from boats, is two tanks: a large one for storage and a small day tank that the engine alone draws from. Each morning you pump from storage to the day tank, passing the fuel through the filter on the way, so that whatever the engine sees has already been cleaned of water and grit. It is the sort of arrangement that costs a little in plumbing and complication and pays for itself the first time you fill up at some remote pump with fuel of doubtful provenance. To have a name and a firm and a willing contact arrive by accident on the last morning, after three days of looking for exactly this kind of thing on purpose, was a quiet pleasure.

We left at half past nine. Five hundred and fifty miles ahead, the SL with its soft top up as it had been the whole trip, the 1997 car carrying us home as steadily as it had carried us out. I had loaded a long playlist days ago, rock and jazz and classical with a little funk threaded through it, and we simply let it run, as we had let it run across the whole journey.

Three stops, to mirror the outbound leg, which had also taken three. The first was fuel: Shell V-Power Racing, the 100RON grade, at 2.80 euros a litre. Dearer even than the fill on the way out, which had been close to 2.50 at a middle stop. I have only ever found that grade in Germany, and so I bought it as a kind of send-off for the old car, a last treat before the duller petrol of home, the price be damned. The second stop was for legs and food, the rolls we had made up at breakfast and wrapped in a napkin, eaten standing by the car. The third, more fuel, at the Belgian and French border.

It was at that second stop that the roof finally came down. The sky had settled into a clean, washed blue and the air had turned properly warm at last, and there seemed no longer any reason to keep the world shut out. Down it went, for the first time all week, and it stayed down the rest of the way into Calais. I had half expected it to be worth having only on the country roads, but it was the opposite: with the top folded away even the plain motorway miles changed character entirely, the noise and the rush of air no longer something to be sealed against but the whole point of the thing, the drive turned from a distance to be covered into something to be in. Nine hundred miles of this trip with the roof up, and the best of the driving came in the last few hours with it down.

The traffic flowed the whole way, which on a return leg through Germany, Belgium and France is not to be assumed. We reached the LeShuttle terminal at Calais at quarter to six, a full hour and a half ahead of check in closure for the train we had booked, and were waved onto an earlier train. Home by half past eight, local time.

Twelve hours, door to door. The same as the journey out, to the hour. There is a tidiness to that which I noticed with more satisfaction than the fact really warrants. Between songs, and again on the last stretch of motorway, Ochi and I came back more than once to the same observation: that it had been a genuinely restful trip and a productive one, which is a rarer combination than it sounds. The Separ conversation is the loose end I want to pull on first. Filters, two tanks, a morning routine of pumping clean fuel forward. A year from now it may be the thing that keeps us moving.