The Karakoram Highway leaves the Indus and ascends the Hunza and Gilgit valleys to Gilgit, centre of Pakistan's mountainous Northern Areas, still notionally under dispute with India. British presence here in the late 19th century gave Gilgit a rich association with the "Great Game" in which Britain and Russia competed for Central Asian influence.

Beyond Gilgit, up the Hunza valley, tongues of glacier spill down the mountains. Little concrete lions stand on the railings of Chinese-built bridges, and the poplars lining the road and irrigation races are turning yellow. Apricot trees have been stripped of their harvest but are still in leaf; and crops of spinach, corn and cabbage thrive in glacial silt. The sediment and everything it touches is grey with mica; and silver-grey water comes from the taps at our hotel in Chalt. Under low cloud which admits no view of the mountains, the sign at a chai house says "Stop here, make snap of Rakaposhi". Two cows misjudge a bluff and tumble 50 metres in a shower of rocks and dust. They pick themselves up and stand motionless until we're out of sight.

But these are diversions. Overriding all is our concern for Apollo's back tyre, and the paranoid feeling, often right, that it is going soft.

A tandem's tyres need high pressure, which in extreme heat means trouble. For the last week Apollo has gathered punctures from tiny thorns and by failure of patches and seams and (most worrying) of side-walls. Already we've made inroads into our companions' spares. An English cyclist in Gilgit gave us a tyre, saving Apollo's riders from abandoning the journey.

In Karimabad, below the Ultar Valley winding skyward to a hint of peak and glacier among cloud, we tour the dusty, cavernous earth-floored rooms of newly-restored Baltit Fort. The Mir (king) of Hunza has a new palace, but until the 1940s his predecessors inhabited this structure of stone, clay and interwoven timbers.

The guide indicates "Mir's reception room", "Mir's storeroom", "Mir's kitchen", "Mir's dining room", "Mir's gun room", "Mir's guest room", and finally, on the topmost level, "Mir's bedroom", with a tall black telephone that looks ready to sprout arms and legs. And outside on the roof, under an awning edged with little turned finials, is the Mir's carpeted sofa whence, in old photographs in books on Hunza, a sepia Mir surveys his kingdom. A slide projector displays a succession of blurred Mirs, the more recent sporting British military rig.

We walk steep morainic hill-sides with terraced corn and spinach and smiling women who give us newly picked apples. At Altit, a second old fort grows straight out of the granite cliff, un-restored and disintegrating and even more charming than the first. Ikramali, aged 11, guides us. The range of rooms is similar, with the addition of "Mir's toilet", which would do an eagle proud. Ikramali laughs at our jokes, and we tell him he will one day be an important man in Pakistan. Which is quite possibly true.

From Karimabad the route climbs through a great gorge like those in New Zealand's Southern Alps. Our friends ahead are dwarfed by an immensity of mountain, and the road is a mere scratch on the cliffs. The rock seems heaved up from the very base of the crust, contorted and shot through again and again with great veins of intrusion.

So we come to Passu, where a glacier creeps down almost to the highway. On foot, we cross a long, fragile suspension bridge and gain the alpine meadows above to look down on a trio of glaciers: Gulkin, Passu, Batura. In the evening Anna and David from Wellington, cycling the other way, lend pliers for a last futile effort to resuscitate our sickly stove, choked with Pakistani carbon. The distances between hotels will soon be more than a day's ride, and without a means of cooking we shall have to depend on whatever charity we can find.

At Sust (3100m), Pakistan's northern frontier town, the mountains press in on all sides. In the only street, with more pedestrians than vehicles, Ann searches for food that can be eaten raw. A hotel manager says he'd gladly join us as expedition cook ("you will need hot soup") but duty keeps him in Sust.

We ride off towards the Khunjerab Pass, still two and a half days away at our plodding pace. The valley narrows to river, rock and road, and we come exhaustedly to Dhi, an outpost manned by the Khunjerab Security Force and staff of the Khunjerab National Park, whose head is a proud sixty and has lost a finger fighting India. The National Park men give us a room and share their cooking fire, fuelled with twigs from the last trees we are to see before China. Wandering the roadside are the Mir of Hunza's cream-brown camels, here for the summer. One of them pauses from nibbling thorny bushes for a tickle under the chin and follows us down the road. They are neither the first nor the last camels we meet but certainly the friendliest, and they cement our love affair with these agreeable beasts.

Our last night in Pakistan is at Koksil. To reach it we gain 700 m up-valley fighting tiredness, for the road is now more than 4000m above sea level. The sole reason for Koksil's existence is the Khunjerab Security Force, whose men supply yak meat cooked on their stove (fired with Chinese coal) and lend us a room and a kerosene burner. The little stream and our water bottles freeze in the night, and the perspiring heat of Rawalpindi seems far indeed.