Iranian diesel may be the cheapest fuel in the known galaxy, but you'd better have decent gloves and a few spare fuel filters.

Watch the stuff in winter too - some of it waxes even when cut with 10% petrol in the northern Iranian night-time temperatures.

From the warmth of the Persian Gulf coast of southern Iran, the road north to the Turkish border saw the temperatures drop. Especially at night. One morning, a local coal-fired boiler had to be robbed of a bucket of glowing embers to get things moving through the fuel system again.

And so into Turkey via the Iranian customs building at Dogubayazit.

With sight of the Carnet document amongst my paperwork, I was ushered into a room occupied singularly by a guy behind a computer screen.

In the same way that I would look severely out of place standing on my head in an empty matchbox, this guy just didn't seem to fit the scene.

Blessed with the eyesight of a bat, he squinted at the screen through both a deep frown and his friend's glasses.

His sole mission within his working day was to input Carnet data into his computer. This entailed circulating his sight from a piece of paper on the desk, to the keyboard, to the computer screen, and back to the piece of paper again. With his glasses, this meant first touching his nose on the paper, then brushing it on the keyboard, nosing up against the computer screen to check the result, and then back to the paper.

He had a little dark patch on the tip of his nose where the screen's transferred static attracted dust from the sheets of paper.

He'd been taught two English words, with which he'd been reassured would deal with any possible situation: "sit" and "down". An authoritative character, he articulately joined these two words into a phrase and repeatedly blasted them across the room, while never once removing his eyes more than three inches from his workface.

One problem - there were no chairs in the room.

After handing him the Carnet, and so as not to appear to disobey his command, it was then a case of going through the motions of slowly wandering around the room as if not quite being able to decide upon which chair to sit, until the tip of his nose was slightly darker, and he handed me a concluding printout.

Back out of the room, after a quick vehicle inspection, it was off into Turkey. One step closer to home.