As Rosa and Doris brought their ambulance in through the gates of the Paddington St John Ambulance station, they saw their boss, Chesney Warren, standing by the door to the building, waving to them to hurry.

‘Something’s happened,’ said Doris, concerned.

They’d just returned to the station from delivering a woman with a suspected burst appendix to the nearest hospital. Rosa stopped the ambulance beside Warren and Doris wound her window down.

‘What is it and where is it?’ she asked, reaching out for the usual piece of paper.

‘Neither,’ said Warren. ‘It’s the BBC on the phone for Rosa.’

‘For me?’ said Rosa in surprise.

‘Yes. I saw you coming in as I picked the phone up and this chap said, “Excuse me, would it be possible to speak to Rosa Weeks? I’m calling from the BBC.” Very posh-sounding.’

‘How did he get this number?’ asked Rosa.

‘I’ve no idea,’ said Warren, ‘but the fact he did suggests he definitely wants to talk to you.’

Rosa got down from the cab and hurried into the building and Chesney Warren’s office, Warren following her. She picked up the receiver and said, ‘This is Rosa Weeks.’

‘I’m very sorry to trouble you at work, Miss Weeks,’ said a cultured but very gentle man’s voice. ‘My name’s John Fawcett and I’m the producer of Henry Hall’s Guest Night on the BBC Light Programme. I know this is very short notice but I wonder if you might be free next Tuesday, the 31st December, New Year’s Eve, and if you might consider appearing on the show.’

Rosa was momentarily stunned into silence, then she said, doing her best to keep her voice controlled, ‘Yes, I would. Thank you.’

‘It is a live broadcast,’ said Fawcett. ‘Next Tuesday we’ll be at the Finsbury Park Empire. The show is broadcast from 6 p.m., but you’d be needed for rehearsals that afternoon, from one o’clock, if that would be convenient for you.’

‘Yes, indeed, that would be convenient.’ She looked at the intrigued Chesney Warren and mouthed, ‘I’ll tell you in a moment,’ as Fawcett said, ‘That is absolutely wonderful, Miss Weeks. Can we meet to discuss your slot on the programme?’

‘Of course. When do you suggest?’

‘Can you be free for lunch tomorrow? The BBC canteen does reasonable fare.’

‘Yes.’

‘Excellent. It’ll have to be at the Maida Vale studios in Delaware Road. We’ve got a few problems at Broadcasting House. Bomb damage, I’m afraid. Do you know Maida Vale?’

‘Yes,’ said Rosa.

‘Very good. I’ll see you there. Will one o’clock suit you?’

‘One o’clock will be fine.’

Rosa replaced the receiver and looked at Chesney Warren, stunned.

‘That was the BBC,’ she said.

‘I know,’ said Warren. ‘I was the one who told you. What did they want?’

‘They want me to appear on Henry Hall’s Guest Night next Tuesday, live from the Finsbury Park Empire.’ Apologetically, she added, ‘He asked me if I would, and also if I’d meet him for lunch tomorrow to discuss my spot, and I said yes without asking you. I’m very sorry.’

‘Nonsense,’ said Chesney. ‘No apology needed. D’you think you’ll be able to get a mention about the St John Ambulance and that you’re a volunteer driver?’

‘I can certainly ask,’ said Rosa. ‘With a war on they might be pleased to tell the audience that.’

Or they might get upset with me for daring to ask and drop me, thought Rosa. However, John Fawcett had sounded the sort of kind man you could ask things without him taking offence. And it wasn’t as if she was demanding something for herself.

The exterior of Down Street had been kept intact despite its closure as a working Underground station; the three-arched semi-circular window above the entrance, the familiar coloured tiles in general use across the London Underground system were fixed to the walls as décor, and above the arch on the right that led to a rear courtyard a sign still read ‘Down Street’. Coburg and Lampson went to the black door to the left and rang the bell. A disembodied voice came from the grille on the right asking what they wanted.

‘DCI Coburg and DS Lampson from Scotland Yard,’ Coburg said to the speaker. ‘We’re here to investigate the recent murder here.’

‘One moment,’ said the voice.

There was a pause, then the door opened and a man in the uniform of London Underground’s railway police looked out at them. Coburg and Lampson showed him their warrant cards, and were admitted.

‘I’ll take you down to see Mr Purslake,’ said the officer.

They followed the uniformed officer to the head of a circular staircase and then down the winding stairs to deep underground. As they descended, they passed different places where there were exit points from the staircase going off into long corridors. It was at one of these exit points that the uniformed officer stopped. A man in a very smart clerical dark grey suit and bow tie was standing waiting for them.

‘The gentlemen from Scotland Yard, Mr Purslake,’ said the officer.

‘Thank you, Wilkins,’ said Purslake. ‘I’ll take over from here.’

The uniformed officer headed back up the stairs, and Purslake held out his hand in greeting to the two policemen.

‘Jeremy Purslake,’ he announced, shaking their hands. Coburg and Lampson introduced themselves to Purslake, who led the way further down the winding staircase.

‘It’s very deep,’ he said, ‘which is why it was chosen.’

‘No lift?’ asked Coburg.

‘There is a two-person lift, but it’s usually kept for senior personnel in case of emergency. I prefer to use the stairs.’ He gave them an unhappy look. ‘This is all most unpleasant. We’ve never had anything like that here before.’

‘How long have you been here?’ asked Coburg.

‘Since last year.’

‘I thought the station had been closed long before that,’ said Coburg.

‘It had,’ said Purslake. ‘Down Street opened as a station in 1907 and closed in 1932.’

‘Why did it close?’

‘Too few passengers were using it. We believe because, as it was in an affluent area, most of the local residents didn’t use public transport but preferred cars and taxis. The other problem was its close proximity to other Tube stations, Green Park, which was originally called Dover Street, and Hyde Park Corner.

‘Once it closed as a working station for passengers in 1932, the tracks were altered and the platform tunnels rebuilt to allow a junction to be installed with access from both the eastbound and westbound services to a new siding located between Down Street and Hyde Park Corner. The siding was mainly for westbound trains to reverse into it, but there was enough space for it to be used for servicing the trains. A small foot tunnel was built from the western end of the siding to Hyde Park Corner station. The lifts were taken out and the lift shafts adapted to provide additional ventilation.

‘Early in 1939 the station was chosen as an underground bunker for the use of the government in the event of war. The station’s great depth means it will be safe against bombing. Brick walls were constructed at the edges of the platforms, and the enclosed platform areas, along with the spaces in the passages, were divided up into offices, meeting rooms and dormitories.

‘The small two-person lift I mentioned earlier was installed in the original emergency stairwell and a telephone exchange and bathrooms were added. It was intended that the Railway Executive Committee would be the sole occupants and from here we’d be able to ensure the trains across the country would keep running. That’s why having the switchboard based at the deepest point is so essential: there will always be telephone connections to everywhere on the rail network. Previously the Railway Executive Committee had been based at Fielden House in Westminster, but the basement there was unsuitable. It wouldn’t have been able to withstand heavy bombing.’

They had now reached the lowest level and Purslake led them along a short section of tunnel to an office. Inside the office, ten people were hard at work at desks, some on telephones, some typing, some sorting through reports. The workers gave the visitors brief glances, then returned to their tasks.

At that moment there was the loud rush of a train hurtling past and the wall vibrating. Lights from the passing train flickered in a metal grille set high in the...