The policeman waited for me to leave, carrying my chair to the wall. Frank embraced me and stood back, still smiling his quirky smile. He picked up his cigarettes and nodded to me.

'Believe me, Charles, I belong here.'

But frank did not belong there. As I left the driveway of the Los Monteros Hotel, joining the coast road to Malaga, I drummed the steering wheel so fiercely that I drew blood from a thumbnail. Neon signs lined the verge, advertising the beach bars, fish restaurants and nightclubs under the pine trees, a barrage of signals that almost drowned the shrill tocsin sounding from the magistrates' court in Marbella.

Frank was innocent, as virtually everyone involved in the murder investigation accepted. His plea of guilty was a charade, part of some bizarre game he was playing against himself, in which even the police were reluctant to join. They had held Frank for a week before bringing their charges, a sure sign that they were suspicious of the confession, as Inspector Cabrera revealed after my meeting with Frank.

If Senor Danvila was the old Spain -measured, courtly and reflective – Cabrera was the new. A product of the Madrid police academy, he seemed more like a young college professor than a detective, a hundred seminars on the psychology of crime still fresh in his mind. At ease with himself in his business suit, he contrived to be tough and likeable, without ever lowering his guard. He welcomed me to his office and then came straight to the point. He asked me about Frank's childhood, and whether he had shown an overlit imagination as a boy.

'Perhaps a special talent for fantasy? Often a troubled childhood can lead to the creation of imaginary worlds. Was your brother a lonely child, Mr Prentice, left by himself while you played with the older boys?'

'No, he was never lonely. In fact, he had more friends than I did. He was always good at games, very practical and down-to-earth. I was the one with the imagination.'

'A useful gift for a travel writer,' Cabrera commented as he flicked through my passport. 'Perhaps as a boy your brother displayed a strain of would-be sainthood, taking the blame for you and his friends?'

'No, there was nothing saintly about him, not remotely. When he played tennis he was fast on his feet and always wanted to win.' Sensing that Cabrera was more thoughtful than most of the policemen I had met, I decided to speak my mind. 'Inspector, can we be open with each other? Frank is innocent, you and I both know that he never committed these murders. I've no idea why he confessed, but he must be under some secret pressure. Or be covering up for someone. If we don't find the truth the Spanish courts will be responsible for a tragic miscarriage of justice.'

Cabrera watched me, waiting silently for my moral indignation to disperse with the rising smoke of his cigarette. He waved one hand, clearing the air between us.

'Mr Prentice, the Spanish judges, like their English colleagues, are not concerned with truth – they leave that to a far higher court. They deal with the balance of probabilities on the basis of available evidence. The case will be investigated most carefully, and in due course your brother will be brought to trial. All you can do is wait for the verdict.'

'Inspector…' I made an effort to restrain myself. 'Frank may have pleaded guilty, but that doesn't mean he actually committed these appalling crimes. This whole thing is a farce, of a very sinister kind.'

'Mr Prentice…' Cabrera stood up and moved away from his desk, gesturing at the wall as if outlining a proposition on a blackboard before a slow-witted class. 'Let me remind you that five people were burned to death, killed by the most cruel means. Your brother insists he is responsible. Some, like yourself and the English newspapers, think that he insists too loudly, and must therefore be innocent. In fact, his plea of guilty may be a clever device, an attempt to unfoot us all, like a…

'Drop-shot at the net?'

'Exactly. A clever stratagem. At first, I also had certain doubts, but I have to tell you that I'm now inclined to think of your brother and guilt in the same context.' Cabrera gazed wanly at my passport photograph, as if trying to read some guilt of my own into the garish photo-booth snapshot. 'Meanwhile, the investigation proceeds. You have been more helpful than you know.'

After leaving the magistrates' court Senor Danvila and I walked down the hill towards the old town. This small enclave behind the beachfront hotels was a lavishly restored theme village with mock-Andalucian streets, antique shops and café tables set out under the orange trees. Surrounded by a stage set, we silently sipped our iced coffees and watched the proprietor scatter a kettle of boiling water over the feral cats that plagued his customers.

This scalding douche, another stroke of cruel injustice, promptly set me off again. Senor Danvila heard me out, nodding mournfully at the oranges above my head as I repeated my arguments. I sensed that he wanted to take my hand, concerned as much for me as he was for Frank, aware that my brother's plea of guilty also involved me in some obscure way.

He agreed, almost casually, that Frank was innocent, as the police's delay in charging him tacitly admitted.

'But now the momentum for conviction will increase,' he warned me. 'The courts and police have good reason not to challenge a guilty plea – it saves them work.'

'Even though they know they have the wrong man?'

Senor Danvila raised his eyes to the sky. 'They may know it now, but in three or four months, when your brother comes to trial? Self-admitted guilt is a concept they find very easy to live with. Files can be closed, men reassigned. I extend my sympathies to you, Mr Prentice.'

'But Frank may go to prison for the next twenty years. Surely the police will go on looking for the real culprit?'

'What could they find? Remember, the conviction of a British expatriate avoids the possibility of a Spaniard being accused. Tourism is vital for Andalucia – this is one of Spain 's poorest regions. Inward investors are less concerned by crimes among tourists.'

I pushed away my coffee glass. 'Frank is still your client, Senor Danvila. Who did kill those five people? We know Frank wasn't responsible. Someone must have started the fire.'

But Danvila made no reply. With his gentle hands he broke his tapas and threw the pieces to the waiting cats.

If not Frank, then who? Given that the police had ended their investigation, it fell to me to recruit a more aggressive Spanish lawyer than the depressed and ineffective Danvila, and perhaps hire a firm of British private detectives to root out the truth. I drove along the coast road to Malaga, past the white-walled retirement complexes marooned like icebergs among the golf courses, and reminded myself that I knew almost nothing about Estrella de Mar, the resort where the deaths had occurred. Frank had sent me a series of postcards from the Club, which portrayed a familiar world of squash courts, jacuzzis and plunge-pools, but I had only the haziest notions of day-to-day life among the British who had settled the coast.

Five people had died in the catastrophic fire that had gutted the Hollinger house. The fierce blaze had erupted without warning about seven o'clock in the evening of 15 June, by coincidence the Queen's official birthday. Clutching at straws, I remembered the disagreeable Guardia Civil at Gibraltar and speculated that the fire had been started by a deranged Spanish policeman protesting at Britain 's occupation of the Rock. I imagined a burning taper hurled over the high walls on to the tinder-dry roof of the villa…

But in fact the fire had been ignited by an arsonist who had entered the mansion and begun his murderous work on the staircase. Three empty bottles containing residues of ether and gasoline were found in the kitchen. A fourth, half-empty, was in my brother's hands while he waited to surrender to the police. A fifth, filled to the brim and plugged with one of Frank's tennis club ties, lay on the rear seat of his car in a side-street a hundred yards from the house.