My memories of the journey I seldom care to reflect upon. The most insistent thing that comes to mind is the soft, yet gritty, sand. A building contractor would have delighted in it because it would be ideal for making high-quality cement and concrete and, no doubt, some sharp entrepreneur will find some way of shipping it out and making a profit. God knows, there's enough of it. But I can never now look upon an expanse of sand without feeling, in imagination, the cruel tug of that damned jerrican on my back. We passed the place where Konti had hamstrung Bailly, crossed the valley and climbed another of the dunes which, in that place, were running from sixty to a hundred feet high. I suppose we were lucky in a way because the forward slopes which we had to climb were not as steep as the reverse slopes. Had we been going north instead of south it would have been much worse.

I watched Byrne going ahead of me across a valley floor and it came to me that there was some significance to that languid, gliding walk of the Tuareg – it came from much walking in sand, using the most economical means possible. I tried to imitate it without much success; you had to have been born with it or trained by the years like Byrne. My feet were more accustomed to city pavements.

We climbed another dune, feet digging into the sand against insistent pull of the back packs, and sometimes slipping backwards. On the crest I paused for breath and looked around. Byrne had well described an erg as a sea of sand. The Tenere was like a still picture of a storm at sea, the waves frozen in mid-heave. But these waves were bigger than any wave of water and stretched interminably as far as I could see.

The sun was setting, casting long shadows into the troughs, and the crest of the dune on which I was standing wound sinuously for many miles until it dipped out of sight. The dunes themselves were soft and smooth, sculptured by the wind, unmarked by footprint, whether of man or animal.

Byrne gestured impatiently and we went slipping and sliding down the other and steeper side. Many times during that awful journey I lost control during these descents. The jerrican on my back would seem to push and I would lose my balance and fall headlong. Luckily the sand was soft and cushiony, but not so soft in individual grains that it wasn't also abrasive, and the skin of my hands became tender.

If I was suffering like that, what of Billson? I had lived a sedentary city life but had tried to temper myself and keep in condition by gymnastics and fencing. Paul had worked for fifteen years in the same dreary office in Luton and, from what I had gathered during the course of investigating his life he hadn't done much to keep fit. But the odd thing was that during this time he didn't complain once. He stolidly climbed and just as stolidly picked himself up when he slipped and fell, and kept up the same speed as the rest of us, which wasn't all that slow with Byrne setting the pace.

I was slowly coming to a conclusion about Paul. Some men may be sprinters, good in the short haul and competent in a crisis. Paul might prove to be the reverse. While not handling crises particularly well he was tenacious and stubborn, as proved by his lifelong obsession about his father, and this stroll across the Tenere was bringing out his best qualities. Be that as it may, he did as well as anyone on that journey, ill-conditioned though his body was for it We stopped on top of a dune just as the sun was dipping below the horizon, and Byrne said, 'Okay; you can take off your packs.'

It was a great relief to get rid of that jerrican which had seemed to increase in weight with every step I took. Billson slumped down and in the red light of the setting sun his face was grey. I remembered that he had been shot in the shoulder not many weeks before, and said gently, 'Here, Paul; let me help you.' I helped him divest himself of his back pack, and said, 'How's your shoulder?'

'All right,' he said dully.

'Let me look at it.' His chest was heaving as he drew panting breaths after that last climb and he made no move, so I unbuttoned the front of his shirt and looked at his shoulder before it became too dark to see. The wound, which had been healing well, was now inflamed and red. It would seem that the pull of the jerrican on the improvised harness was chafing him. I said, 'Luke, look at this.'

Byrne came over and inspected Paul He said, 'We drink the water out of his can first.'

'And perhaps we can transfer some into my can.'

'Maybe,' he said noncommittally. 'Let's eat.'

Our dinner that night was cold and unappetizing. The stars came out as the light ebbed away in the west and the temperature dropped. Byrne said, 'Better wear djellabas.'

As I put mine on I asked, 'How far have we come?'

'Mile and a half – maybe two miles.'

'Is that all?' I was shattered. It seemed more like five or six.

'More'n I expected.' Byrne nodded towards Billson. 'I thought he'd hold us up. He still might. I suggest you take some of his water. Do it now before we leave.'

'Leave! You're not going on in the dark?'

'Damn right I am. We're in a hurry. Don't worry; I have a compass and the moon will rise later.'

I put half of Billson's water into my jerrican, reflecting that Byrne was still carrying a full one. He gathered us together. 'We're moving off now. So far you've not done much talking. That's good because you needed your breath. But now you talk because it's dark – you don't lose contact with anyone and you don't let them lose contact with you. It'll be slow going but we need every yard we can make.'

He said something to Konti, probably repeating what he'd told us, then we descended from the top of the dune. It was damned difficult in the dark, and Byrne kept up a constant grunting, 'Ho! Ho! Ho!', sounding like a demented Santa Claus. But it was enough to let us know where he was, and I was encouraged to raise my own voice in song.

At the bottom he rounded us up and we set off across the valley floor under those glittering stars. I sang again; a ditty from my army days:

'Uncle George and Auntie Mabel Fainted at the breakfast table. Let this be an awful warning Not to do it in the morning.'

I paused. 'Billson, are you all right?'

'Yes,' he said wearily. 'I'm all right.' From the left Konti made a whickering noise. He sounded like a horse. Byrne grunted, 'Ho! Ho! Ho!'

'Ovaltine has put them right, Now they do it morn and night; Uncle George is hoping soon To do it in the afternoon. Hark the Herald Angels sing, "Ovaltine is damned good thing."'

Billson made the first attempt at a witticism that I had heard pass his lips. 'Were you a Little Ovaltiney?'

I bumped into Byrne. 'Now we've had the commercial,' he said acidly. 'Let's get climbing.'

So up we went – slowly.

I didn't know then how long we stumbled along in the dark but it seemed like hours. Later Byrne said he'd called a halt just before midnight, so that meant a six-hour night march at probably not more than half a mile an hour. He stopped unexpectedly when we were half-way up a slope, and said, This is it. Dig in.'

Thankfully I eased the jerrican from me and massaged my aching shoulders. In the light of the moon I saw Billson just lying there. I crawled over to him and helped him out of his harness, then made sure his djellaba was wrapped around him, and built up a small rampart of sand on the downhill side of him to prevent him rolling to the bottom in his sleep. Before I left him he had passed out.

I crawled over to Byrne and demanded in an angry whisper, 'What the hell's the flaming hurry? Paul's half dead.'

'He will be dead if we don't get to where we're going by nightfall tomorrow,' said Byrne unemotionally.

'What do you mean?'

'Well, an azelai don't stop at sunset like we've usually been doing. Mokhtar will push on until about eleven every night. 'Course, it's easy for them, they're going along the valley bottoms.'