The torrential rain, though, put him – and everyone else – back to square one, for set-up was now a matter of guesswork.

At the start Senna was smoothly away, without too much wheelspin, but into the first corner there was black and gold, rather than the expected red and white, in his mirrors. De Angelis had beaten Prost away, and Rosberg was left on the grid, stalled.

At the end of lap the Lotuses came through 1-2, followed by Prost, Alboreto, Warwick and Niki Lauda. Ayrton was treading warily, but just doing it way faster than anyone else. Making the most of his clear vision, he was already lapping at a speed beyond his team mate; after two laps he was three seconds up on Elio.

If one Brazilian looked on course already for victory, another faced perhaps the most dispiriting afternoon of his racing life. For the first three laps Nelson Piquet’s Pirelli-shod Brabham somehow resisted Stefan Johansson’s Ferrari, but a queue was forming up behind them. The Pirelli wets, it was clear, were dire, no match for the Goodyears.

Piquet would stop many times for new tyres, and on the last occasion surprised his mechanics, let’s say, by alighting from his vehicle, and disappearing into the pit. A little while later he smilingly reappeared, now in clean and dry overalls. There was, after all, no hurry, was there?

Eventually Nelson called it a day, as had Jacques Laffite, who parked his Ligier after 16 laps, shrugging that the conditions were absurd, that the race should have been stopped, as at Monte Carlo a year earlier.

Back then, of course, there was no safety car, no facility for continuing the race ‘under yellow’ until the weather improved. And Laffite was not alone in his views. Afterwards Lauda would complain vociferously about the conditions, and even Senna frequently gesticulated to officials that they should call a halt to the race.

They didn’t, though, even when Rosberg crashed at the long right-hander before the pit straight, his car coming to rest in the middle of the track. Keke, his right thumb broken, quickly scrambled out of the cockpit, and ran to safety, but there were some terrifying moments as drivers swerved around the beached Williams.

Far and away the most imperturbable man on the circuit appeared to be the leader. After 30 laps, Senna led by more than half a minute, and Prost continued to crowd de Angelis for second position, with Alboreto’s Ferrari not far behind them. As Prost pounded down the pit straight to begin lap 31, the McLaren suddenly began to weave, veering first left, then right, then breaking into a spin. Alain could do nothing to keep it from hitting the wall: “It was raining very hard just then, with deep puddles, and in the spray it was impossible to see where they were. I was doing maybe 290kph, and once you start aquaplaning at that speed, you are finished…”

A little while later, Alboreto found a way past de Angelis, and Patrick Tambay’s Renault, too, went by the Lotus. After 67 of the originally scheduled 69 laps, the two-hour mark was passed, and the chequered flag went out to Senna. Even before reaching the first turn he had flung off his belts and was half out of the car in his joy.

As he talked about the race later, though, Ayrton’s exuberance had given way entirely to sober reflection. ‘The big danger,” he thought, “was that conditions changed all the time. Sometimes the rain was very heavy, sometimes not. I couldn’t see anything at all behind me. Once I nearly spun in front of the pits, like Prost, and I was lucky to stay on the road.

“Maybe people think I made no mistakes, but that’s not true — I’ve no idea how many times I went off! On one occasion I had all four wheels on the grass, totally out of control… but the car came back on the circuit. Everyone said, ‘Fantastic car control!’ It was just luck…”

Eight years later, in similar conditions, Senna would win for McLaren at Donington, and many suggested it was his greatest drive. Ayrton snorted at that: “No way! I had traction control! OK, I didn’t make any mistakes, but the car was much easier to drive. It was a good win, but, compared with Estoril ’85, it was nothing, really…”

Watching him that day in Portugal, I was sure he was taking the first turn at least one gear higher than usual, and later I asked him about it. “Of course I was!” he retorted. “Less wheelspin… easier to drive. Wasn’t everyone doing that?” Well, no, actually.