‘It’s not over yet,’ he said cheerfully.

Judy was entranced by everything she saw at the Zoological Gardens, although she did feel sad for the loss of freedom that the large animals must surely suffer, even though they were obviously well fed and cared for by their keepers. She and Rob strolled around taking in the sights for over an hour, but the sky began to darken and there was a hint of sulphur in the air, and a yellow tinge to the fog that had begun to creep up on them.

Judy had experienced this only too often as a child. ‘It’s the beginning of a peasouper,’ she said urgently. ‘I think we’d better leave, Rob. When it gets dark you won’t be able to see your hand in front of your face, quite literally.’

‘Yes, I was afraid of that. We’d better cut our visit short or we might be stuck in London overnight.’

They left the gardens but it seemed that the whole of the West End had realised what was happening, and everyone was making for home or shelter of some kind. Cabs rushed past them, but all were booked, and the air grew steadily thicker as the fog swirled around them. Eventually Rob managed to flag down a hackney carriage, but the roads were so congested that they were travelling at a snail’s pace. The gas lamps were being lit and darkness was falling. The heavy cloud cover had forced the smoke from both domestic chimneys and manufactories to mix with the fog and had formed a choking yellow London Particular. Eventually the cab came to a halt.

Rob opened the door and leaned out. ‘Where are we, Cabby?’

‘I’m sorry, mister. This is the best I can do for you. We’re at Charing Cross Station and with luck I’ll make it to the cabmen’s shelter at St Clement Danes church.’

‘Is there any chance you could get us as far as Bishopsgate Station? I’ll pay double if you can.’

‘I’d like to oblige, but we’d never make it. The best you can do is to put up here, at the Charing Cross Hotel. You won’t get no further, I promise you.’

Judy caught Rob by the sleeve. ‘We could try to walk there.’

‘It’s too far. The cabby’s right, Judy. The fog is getting thicker by the minute. We haven’t got much choice.’ Rob alighted from the cab, holding his hand out to Judy.

She could only just make out his silhouette as the thick, choking pea-green fog closed in on them, but his fingers closed around hers in a warm clasp and a moment later she was standing beside him on the cobblestones. He paid the cabby and the vehicle moved off slowly, edging its way along the Strand, and was lost within a few feet of them.

‘We haven’t any choice,’ Rob said, linking her hand through the crook of his arm. ‘We’ll have to stay here tonight.’

Judy shivered. ‘You’re right. We’d never find our way in this, and it’s a long way from here to Bishopsgate.’