The man glanced at the photograph. ‘Yes, that’s her. It’s the same photo that was in the papers, isn’t it?’

‘Yes, I’m just making sure. The reproduction is sometimes a bit poor.’

In the aquarium itself, red-eared terrapins basked on a concrete beach under halogen spot lamps. There were vegetarian piranhas and South American snakeheads that swam with their long bodies hanging in graceful curves, ready to travel across land to find more water if their habitat dried up. One glass tank contained only the hands of an unseen man scraping the silt off the bottom.

‘She came in on Saturday afternoon, about two fifteen. Didn’t stop here for long. She had a quick walk round, then spent a few minutes by the thermal pool.’

‘What made you notice her?’

‘Well, it’s not often we have people coming in on their own. A woman of sixty or so? She seemed harmless, but you never know.’

‘So you kept an eye on her?’

‘Discreetly. On my own initiative.’

‘What did she do?’

‘Pretty much what you did – stopped at the petrifying well, then came through here and looked at the fish. She liked the terrapins, as I recall.’

He opened a door, and they walked out of the heat of the aquarium on to a cool, tiled walkway beside an outdoor pool.

‘This was the original baths, you know. The Victorians thought the water cured rheumatism. The pool is still fed by the thermal spring. It’s a constant twenty degrees centigrade.’

Cooper looked over the side of the pool. It was full of colourful Japanese fish and fat mirror carp.

‘Twenty degrees centigrade? Lucky fish.’

‘The lady stood out here for a bit. There were some children feeding the fish. See? You can buy a carton of food from the machine for twenty pence.’

‘Did she speak to anyone?’

‘No, she was just watching.’

Despite the temperature of the water, it was much cooler out here, because the pool was open to the sky, with only a few rusty girders remaining of the roof. Looking up, Cooper noticed a camera mounted on the wall, focused on the feeding station.

‘Are you sure? No one standing near her? Or someone that she took a particular interest in?’

‘Only the children.’

Cooper waited on the red and black tiles while the man walked off to check a side door. There was a plop as one of the larger fish surfaced at the other end of the pool. And then he saw that people had thrown coins in here, too. Instead of buying food for the fish, visitors had tossed their twenty pence pieces into the water, so they lay glinting on the bottom.

‘We go out this way. Through the hologram gallery.’