Back at the start of World War Two the authorities forbade the use of the Underground as an air-raid shelter. Instead Londoners were supposed to rely on hastily built neighbourhood shelters or on the famous Anderson shelters which were basically rabbit hutches made from corrugated iron with some earth shovelled on top. Londoners being Londoners, the prohibition on using the Underground lasted right up until the first air-raid warning, at which point the poorly educated, but far from stupid, populace of the capital did a quick back-of-the-envelope comparison between the stopping power of ten metres of earth and concrete and a few centimetres of compost, and moved underground en masse. The authorities were appalled. They tried exhortation, persuasion and the outright use of force but the Londoners wouldn’t budge. In fact, they started to organise their own bedding and refreshment services.

And thus in a cloud of official disapproval the Blitz spirit was born.

A couple of thousand preventable deaths later, the government authorised the construction of new purpose-built deep-level shelters constructed, according to Kumar, using the same techniques and machines as the tube system itself.

I knew all about the shelters at Belsize Park and Tottenham Court Road – it’s not like you can miss the huge fortified concrete pillboxes that marked the ventilation shafts – but I’d never heard of one at Holland Park.

‘There used to be a top-secret government agency down here,’ said Kumar. ‘Only I heard they got relocated to Scotland.’

The opposite end of the tunnel was far enough away to be in shadow. I was tempted to brighten my werelight, but I was getting worried about the amount of magic I’d been using. Dr Walid’s guidelines, endorsed by Nightingale, were that I should refrain from doing more than an hour of continuous magic if I wanted to avoid what he called thaumatological necrosis and me and Lesley called cauliflower brain syndrome.

‘They did a good job stripping this place then,’ I said. It was completely empty. I could even see where light fittings had been prised out of the concrete walls. The bass rumble was louder, but it was hard to tell from where it was coming.

‘This is the intersection,’ said Kumar.

You could see the circular outline where a tunnel of similar size to ours had formed a crossroad and then been walled off with concrete and cement. There were four doors on each side, two at our ground level and two halfway up the wall servicing a floor level that had either been stripped out or never installed.

The doors were normal sized, but made of steel with no obvious handles on our side.

‘Left or right?’ asked Reynolds.

I put my ear against the cold metal of the nearest door – the bass rumble was loud enough for me to identify the track.

‘“Stalingrad Tank Trap”,’ I said. ‘By Various Artiz.’