'Now…' Cabrera turned his back on the heavy oak doors, still sealed by their police tapes, and pointed to the gravel path that led around the south-west corner of the mansion to the kitchen and garages. 'The hall and the downstairs rooms are too dangerous for anyone to enter. I suggest therefore that we follow the path taken by the assassin. This way we will see events in the order they took place. It will help us to understand what happened on that evening and, perhaps, read the psychology of the victims and their murderer Enjoying his new role as stately home tour guide, Cabrera led us around the house. The Hollingers' blue vintage Bentley stood outside the garage, the only clean and polished entity in the estate. Miguel had washed and waxed the limousine, buffing the huge wings that flared above its wheels. He had followed us up the drive and now stood patiently beside the car. Still staring at me, he took up his leather and began to rub the high radiator grille.

'Poor man…' Paula held my arm, eyes avoiding the debris of charred timbers around us, and tried to smile at the chauffeur. 'Do you think he's all right here?'

'I hope so. It looks as if he's waiting for the Hollingers to return. Inspector, what do you know about the psychology of chauffeurs…?'

But Cabrera was pointing to the flight of steps that led to the housekeeper's quarters above the garage. A wrought-iron gate beside the entry door opened on to the blighted hillside where the lemon orchard had once flourished. Cabrera sprinted up the steps and stood by the gate.

'Dr Hamilton, Mr Prentice… you will notice the gate from the orchard. We can now imagine the situation on the evening of 15 June. By seven o'clock the party is well in progress, with all the guests on the terrace beside the pool. As Dr Hamilton will remember, the Hollingers appear on the top-floor veranda and propose the toast to your Queen. Everyone is looking up at the Hollingers, glasses raised to Her Majesty, and no one notices the arsonist when he opens the orchard gate.'

Cabrera skipped down the staircase and stepped past us to the kitchen entrance. He took a set of keys from his pocket and unlocked the door. I appreciated his tactful use of the term 'arsonist', though I suspected that the tour was designed to rid me of my faith in Frank's innocence.

'Inspector, Frank was mingling with the other guests on the terrace. Why would he leave the party and hide in the orchard? Dozens of people talked to him by the pool.'

Cabrera nodded his sympathy at this. 'Exactly, Mr Prentice. But no one remembers speaking to him after six-forty-five. Did you, Dr Hamilton?'

'No – I was with David Hennessy and some friends. I didn't really see Frank at all.' She turned away and stared at the gleaming Bentley, pressing the faded bruise on her mouth as if willing it to return. 'Perhaps he didn't go to the party?'

Cabrera firmly dismissed this. 'Many witnesses spoke to him, but none after six-forty-five. Remember, the arsonist needed to park his incendiary materials close at hand. No one would notice a guest walking up the steps to the orchard. Once there, he collects his bombs-three bottles filled with ether and gasoline that he buried the previous day in a shallow excavation twenty metres from the gate. As the Queen's toast begins he sets off to enter the house. The nearest and most convenient door is this one into the kitchen.'

I nodded at all this, but asked: 'What about the housekeeper? Surely she would have seen him?'

'No. The housekeeper and her husband were by the terrace, helping the temporary staff to serve the canapes and champagne. They were raising their glasses to Her Majesty and saw nothing.'

Cabrera pushed back the door and beckoned us into the kitchen. A brick-built annexe to the main house, it was the only part of the structure undamaged. Saucepans hung from the walls, and the shelves were crowded with spices, herbs and bottles of olive oil. But the yeasty stench of sodden carpets and the acrid odour of charred fabrics filled the room. Pools of water lay on the floor, soaking a walkway of wooden boards laid down by the police investigators.

Cabrera waited until we found our footing and then continued. 'So: the arsonist enters the empty kitchen and seals the door. The house is now closed off from the outside world. The Hollingers preferred their guests not to enter their home, and all the doors on to the terrace were locked. In fact, the guests were completely segregated from their hosts – an English tradition, I assume. Now, let us follow the arsonist when he leaves the kitchen…'

Cabrera led the way into the pantry, a large room that occupied a section of the annexe. A refrigerator, washing-machine and spin-drier stood on the concrete floor, surrounded by pools of water. Beside a deep-freezer cabinet an inspection door opened into a head-high chamber that contained the central heating and air-conditioning system. Cabrera stepped through the inspection door and pointed to the partly dismantled unit, a furnace-sized structure that resembled a large turbine.

'The intake manifold draws air from a pipe on the kitchen roof. The air is filtered and humidified, then either cooled during the summer or heated during the winter, and finally pumped to the rooms around the house. The arsonist switches off the system-for the few minutes he needs no one will notice. He removes the cowling of the humidifier, drains off the water and refills the reservoir with his gasoline and ether mixture. All is now set for a huge and cruel explosion Cabrera ushered us through the service corridor that led into the mansion. We stood in the spacious hall, looking at ourselves in the foam-smeared mirrors, visitors to a marine grotto. The staircase rose a dozen steps, then divided beneath a large baronial fireplace that dominated the hall. Tags of scorched cloth lay in the iron grate, and the ashes had been carefully sifted by the police investigators.

Watching me closely, Cabrera continued: 'Everything is ready. The guests are busy with their party outside, eager to drink the last of the champagne. The Hollingers, their niece Anne, the Swedish maid and the secretary, Mr Sansom, have retired to their rooms to escape the noise. The arsonist takes his remaining bottle of gasoline and ether and soaks a small carpet he has laid in the fireplace. The flames leap swiftly from his match or lighter. He then returns to the pantry. As the fire on the staircase begins to rage he switches on the air-conditioning system…'

Cabrera paused, waiting as Paula gripped my arm to steady herself, knuckles between her teeth.

'It's terrible to imagine. Within seconds the gasoline mixture is driven through the ventilation ducts. Ignited by the flames on the staircase, the rooms of the upper floor turn into an inferno. No one escapes, even though there are emergency steps that lead down to the terrace from a door on the landing. Mr Hollinger's film library, in a room next to his study, adds to the blaze. The house is a furnace, and everyone inside it perishes in a matter of minutes.'

I felt Paula stumble against me, and put an arm around her trembling shoulders. She had begun to cry to herself, and her tears stained the lapel of my cotton jacket. She seemed about to collapse, but rallied herself and murmured: 'Good God, who could do such a thing?'

'Certainly not Frank, Inspector.' Still supporting Paula, I spoke as calmly as I could to Cabrera. 'This dismantling of the air-conditioning system and turning it into an incendiary weapon – Frank could barely change a light-bulb. Whoever carried out this attack had the skills you find only in people trained as military saboteurs.'

'Perhaps, Mr Prentice…' Cabrera watched Paula with obvious concern, and offered her his handkerchief. 'But skills can be learned, especially dangerous ones. Let us continue our tour.'