At the end of the lawn was another pill, a narrow gorge twelve feet deep, running along the side of a copse. Suddenly I realised that this was where I had been hurled ashore. Since I had crawled out of the slime into this thicket and gone straight for the forest I had never seen Bullo Pill and its cottages.

Marrin indeed had found privacy here. It seemed to be a better spot for personal meditation than Broom Lodge, here where the clock of one’s life would be governed only by the ebb and flow of the Severn, which ran smoothly down this reach at its game of pretending to be one of the great navigable highways from Europe to ocean. There could be no doubt which was Marrin’s boat: a ten-foot dinghy with an outboard motor lying in the pill along with a salmon boat and a decaying motor cruiser, all laid up on a terrace of mud waiting for the tide to lift them. The dinghy was too small to be used for diving and confirmed my growing opinion that he always went in off the land.

A woman came out of one of the cottages and crossed the head of the pill. After some conversation in which I congratulated her on living in so lovely a spot – she was well aware of it, bless her, with no complaints of isolation! – I asked her if the dinghy was hers. No, she said, it belonged to a gentleman from up the Forest who used it just for crossing the river. She giggled that she supposed he had a friend in Arlingham and slipped over to see her without going all the way round by Gloucester.

That was easy enough. Marrin could reach his dinghy at half-tide, and go down on the ebb when the channel would sweep him away to the opposite bank. There he would have to wait for the flood before returning to Bullo Pill – a matter of anything up to eight hours. What did he do meanwhile? Whatever it was – digging or diving or merely collecting – took place on the other side of the Severn.

There was another useful inference to be drawn. If he wanted to be back at Broom Lodge at or before dawn he would have to choose a night when five or six hours were left in the ebb, wait till the bore had passed and grab the short, favourable tide flowing up behind it.

After a night of dozing rather than sleeping I woke up impatient for action, the more so as there seemed no chance of any at all. The business of staying dead was becoming a bore, and my determination to destroy Marrin one way or another was weakened by the thought of its effect on Elsa. After taking the walk out to the sapling and back in case the major had left a message – sure that if he had it would be crazily impractical – I spent the day like a frustrated housewife, adding twigs to my bed and tying them down, stopping a drip from the iron roof and eating two large meals to make up for a deal of exercise on an empty stomach. A party of campers walked round two sides of my copse but never attempted to enter it, confirming my opinion that it was a safe refuge. But a refuge for what?

Next day at least one mystery was solved, though of little importance. For want of anything better to do I called at Bream post office. It was unlikely that I could already have a reply from the department of zoology, but the letter was waiting for me. Evidently the beast was so well known that my colleague had not needed to refer to library or museum.

‘Your photographs are of a magnificent specimen of glyptodont. Though the solid shell over the back does resemble that of a turtle, the plates over head and tail are segmented. Bones have rotted away, as one would expect, but carapace and armour are largely intact, preserved, I think, by being buried in silt. Microscope will show. When can I see it? Glyptodont is not a reptile but an extinct mammal related to the armadillo and – to judge by the skull – comparatively intelligent. Yours was a young animal, a half-grown glyptodont kitten shall we say? The spiked tail may have been used for smashing into termite nests or as a mace for walloping predators while they tried to get their teeth into the armour. So far as we know, the animal existed only in America and may well have been contemporary with man, like the extinct giant sloth. Where was it found? Come back quickly and tell us. If in this country, the discovery is unique.’

Silt. Another clue to the Severn. When I asked Marrin about the supposed turtle he gave me the unsatisfactory answer that he had set up the remains in his laboratory for fun. He never said that he himself had found it underwater or dug it up. Why not? Probable explanation: because it came from a site which he wished to keep secret.

So off again to the stump of the sapling. The major was there waiting for me, and I was not too pleased to see him, preferring written messages which gave me time to think – very necessary when dealing with a wandering friar, as he had called himself, who was outwardly shrewd but a preposterous visionary inside, so that it was doubtful if even he knew what was going on at the interface where they met.

He had been waiting a couple of hours. His moustache had perked up and his watery blue eyes had dried to a sparkle.

‘Got an idea, Piers!’ he exclaimed. ‘Brilliant! But tell me if you don’t think so. Remember how Simeon left me alone to take those photographs? All stems from that. Shocking breach of trust, but for his own good. And he needn’t know. Make it plain it was burglary.’

Since my mind was running on glyptodonts and tides, I translated these enthusiastic explosions as referring to Marrin’s diving kit or the tools in the back of his van. The object of burglary turned out to be the golden cauldron. The major was proposing to steal it and hand it to me for submission to some authority who could tell us whether it was ancient work or not and, if it wasn’t, whether it had any chance of being sold as such.

‘He’ll be out fishing tonight,’ the major said. ‘So we have to hurry. Last-minute plan. Often effective. Catches ’em on the wrong foot.’

‘How do you know he’ll be out?’

‘Simple. Been watching him. Saw him loading the gear into his van.’

‘But he’s careful not to be observed.’

‘All of them at work except me. After dark I’ll burst in and grab the bowl for you. Then you take the first train up to London and you could be back with it in the afternoon.’

It was very likely that he would be out either on that night or the next. The tide fitted the crossing from Bullo in the late evening and the return before dawn.

However, I doubted my ability to charge into the British Museum, dressed as I was, with a valuable object of gold, and avoid immediate arrest. It depended whether I could get an introduction and a guarantee of good faith in the short time available. I doubted still more if the major could commit a fake burglary without being caught.

‘Nothing to it if I don’t slip up somewhere,’ he said. ‘And if I do, I’ve got a dozen stories to explain what I was up to.’

His scheme as he presented it seemed as if it had been conceived under the round table with an intoxicated Arthur; but the more he spluttered, the more I saw that it was quite likely to work. Burglars, I was sure, had never been seriously considered by the commune. Outsiders knew nothing of Marrin’s products. There were no other valuables, and no burglar would risk breaking in when people were awake and about at all hours.

The lab was at the back of the east wing. Windows were set high up so that no passers-by could see in, and the window at the north end facing the forest was not overlooked by any of the bedrooms. Access was from the estate office by a door which was locked. The key, the major assured me, was kept in a drawer of Marrin’s desk – on the face of it a casual and too confident arrangement, but the hold Marrin had over his colonists must be remembered. None of them would have entered the holy of holies unless invited.

I asked the major how he was going to make such a simple job look like a burglary from outside.