He leaned against my windshield, a greasy hand smearing the glass, and saluted the young woman, who was turning into the police car park beside the checkpoint. He glared down at me, nodding to himself and clearly convinced that he had caught a lecherous tourist in the act of visually molesting the wife of his commanding officer. He moodily flicked through the pages of my passport, unimpressed by the gallery of customs stamps and visas from the remotest corners of the globe. Each frontier crossing was a unique transaction that defused the magic of any other.

I waited for him to order me from the car and carry out an aggressive body-search, before settling down to dismantle the entire Renault until it lay beside the road like a manufacturer's display kit. But he had lost interest in me, his spare eye noticing a coach filled with migrant Moroccan workers who had taken the ferry from Tangier. Abandoning his search of the van and its cargo of dolls, he advanced upon the stoical Arabs with all the menace and dignity of Rodrigo Diaz out-staring the Moors at the Battle of Valencia.

I followed the van as it sped towards La Linea, rear doors swinging and the dolls dancing together with their feet in the air. Even the briefest confrontation with police at a border crossing had the same disorienting effect on me. I imagined Frank in the interrogation cells in Marbella at that very moment, faced with the same accusing eyes and the same assumption of guilt. I was a virtually innocent traveller, carrying no contraband other than a daydream of smuggling my brother across the Spanish frontier, yet I felt as uneasy as a prisoner breaking his parole, and I knew how Frank would have responded to the trumped-up charges that had led to his arrest at the Club Nautico in Estrella de Mar. I was certain of his innocence and guessed that he had been framed on the orders of some corrupt police chief who had tried to extort a bribe.

I left the eastern outskirts of La Linea and set off along the coast road towards Sotogrande, impatient to see Frank and reassure him that all would be well. The call from David Hennessy, the retired Lloyd's underwriter who was now the treasurer of the Club Nautico, had reached me in my Barbican flat the previous evening. Hennessy had been disturbingly vague, as if rambling to himself after too much sun and sangria, the last person to inspire confidence.

'It does look rather bad… Frank told me not to worry you, but I felt I had to call.'

'Thank God you did. Is he actually under arrest? Have you told the British Consul in Marbella?'

' Malaga, yes. The Consul's closely involved. It's an important case, I'm surprised you didn't read about it.'

'I've been abroad. I haven't seen an English paper for weeks. In Lhasa there's not much demand for news about the Costa del Sol.'

'I dare say. The Fleet Street reporters were all over the club. We had to close the bar, you know.'

'Never mind the bar!' I tried to get a grip on the conversation. 'Is Frank all right? Where are they holding him?'

'He's fine. On the whole he's taking it well. He's very quiet, though that's understandable. He has a lot to think over.'

'But what are the charges? Mr Hennessy…?'

'Charges?' There was a pause as ice-cubes rattled. 'There seem to be a number. The Spanish prosecutor is drawing up the articles of accusation. We'll have to wait for them to be translated. I'm afraid the police aren't being very helpful.'

'Do you expect them to be? It sounds like a frame-up.'

'It's not as simple as that… one has to see it in context. I think you should come down here as soon as you can.'

Hennessy had been professionally vague, presumably to protect the Club Nautico, one of the more exclusive sports complexes on the Costa del Sol, which no doubt depended for its security on regular cash disbursements to the local constabulary. I could well imagine Frank, in his quizzical way, forgetting to slip the padded manila envelope into the right hands, curious to see what might result, or omitting to offer his best suite to a visiting commandant of police.

Parking fines, building-code infringements, an illegally-sited swimming pool, perhaps the innocent purchase from a dodgy dealer of a stolen Range Rover – any of these could have led to his arrest. I sped along the open road towards Sotogrande, as a sluggish sea lapped at the chocolate sand of the deserted beaches. The coastal strip was a nondescript plain of market gardens, tractor depots and villa projects. I passed a half-completed Aquapark, its excavated lakes like lunar craters, and a disused nightclub on an artificial hill, the domed roof resembling a small observatory.

The mountains had withdrawn from the sea, keeping their distance a mile inland. Near Sotogrande the golf courses began to multiply like the symptoms of a hypertrophied grassland cancer. White-walled Andalucian pueblos presided over the greens and fairways, fortified villages guarding their pastures, but in fact these miniature townships were purpose-built villa complexes financed by Swiss and German property speculators, the winter homes not of local shepherds but of Dusseldorf ad-men and Zurich television executives.

Along most of the Mediterranean's resort coasts the mountains came down to the sea, as at the Côte d'Azur or the Ligurian Riviera near Genoa, and the tourist towns nestled in sheltered bays. But the Costa del Sol lacked even the rudiments of scenic or architectural charm. Sotogrande, I discovered, was a town without either centre or suburbs, and seemed to be little more than a dispersal ground for golf courses and swimming pools. Three miles to its east I passed an elegant apartment building standing on a scrubby bend of the coastal road, the mock-Roman columns and white porticos apparently imported from Las Vegas after a hotel clearance sale, reversing the export to Florida and California in the 1920s of dismantled Spanish monasteries and Sardinian abbeys.

The Estepona road skirted a private airstrip beside an imposing villa with gilded finials like a castellated fairy battlement. Their shadows curved around a white onion-bulb roof, an invasion of a new Arab architecture that owed nothing to the Maghreb across the Strait of Gibraltar. The brassy glimmer belonged to the desert kingdoms of the Persian Gulf, reflected through the garish mirrors of Hollywood design studios, and I thought of the oil company atrium in Dubai that I had walked through a month earlier, pursuing my courtship of an attractive French geologist I was profiling for L'Express.

'The architecture of brothels?' she commented when I told her of my longstanding plans for a book during our rooftop lunch. 'It's a good idea. Rather close to your heart, I should think.' She pointed to the retina-stunning panorama around us. 'It's all here for you, Charles. Filling-stations disguised as cathedrals…'

Could Frank, with his scruples and finicky honesty, have chosen to break the law on the Costa del Sol, a zone as depthless as a property developer's brochure? I approached the outskirts of Marbella, past King Saud's larger-than-life replica of the White House and the Aladdin's cave apartments of Puerto Banus. Unreality thrived on every side, a magnet to the unwary. But Frank was too fastidious, too amused by his own weaknesses, to commit himself to any serious misdemeanour. I remembered his compulsive stealing after we returned to England, slipping corkscrews and cans of anchovies into his pockets as we trailed after our aunt through the Brighton supermarkets. Our grieving father, taking up his professorial chair at Sussex University, was too distracted to think of Frank, and the petty thefts forced me to adopt him as my little son, the sole person concerned enough to care for this numbed nine-year-old, even if only to scold him.

Luckily, Frank soon outgrew this childhood tic. At school he became a wristy and effective tennis player, and sidestepped the academic career his father wanted for him, taking a course in hotel management. After three yean as assistant manager of a renovated art deco hotel in South Miami Beach he returned to Europe to run the Club Nautico at Estrella de Mar, a peninsular resort twenty miles to the east of Marbella. Whenever we met in London I liked to tease him about his exile to this curious world of Arab princes, retired gangsters and Eurotrash.