“Apparently it’s a mathematical term, sir: 341 is described as a brilliant number because it has two prime factors, eleven and thirty-one. Multiply them and you get 341. Numbers with two prime factors of equal length are known as brilliant numbers and are used in cryptography. Apparently it’s also the sum of the squares of the divisors of sixteen, but I think he was more impressed by the two prime factors. He had no trouble with ‘UDG.’ It stands in his mind for ‘U Done Good’—seems appropriate, sir.”

Dalgliesh said, “The maths means nothing to me, but we must hope he’s right. I suppose we can find someone to confirm it.”

“I don’t think we need bother, sir. He’s just got a first in mathematics at Oxford. He said he can never get stuck behind another vehicle without mentally playing about with the registration number.”

“And the car owner?”

“A bit surprising, on the face of it. It’s a clergyman. The Reverend Michael Curtis. Lives in Droughton Cross. St. John’s Church Vicarage, 2 Balaclava Gardens. It’s a suburb of Droughton.”

The Midlands industrial city could be reached in little over two hours on the motorway. Dalgliesh said, “Thank you, Sergeant. We’ll go on to Droughton Cross as soon as we’ve finished here. The driver may have nothing to do with the murder, but we need to know why that car was parked by the stones and what, if anything, he saw. Is there anything else, Sergeant?”

“One find by the SOCOs, sir, before they left. It’s more odd than significant, I’d say. It’s a bundle of eight old postcards, all of foreign views and all dated 1993. They’ve been cut in two, with the address on the right-hand side missing, so there’s no way of knowing who the recipient was, but they read as if written to a child. They were wrapped tightly in silver paper inside a plastic bag and buried by one of the Cheverell Stones. The SOCO concerned was pretty sharp-eyed, and he saw some evidence that the grass had been disturbed, although not recently. It’s difficult to say what connection they could have with Miss Gradwyn’s death. We know someone was at the stones with a light that night, but if they were looking for the cards they didn’t find them.”

“Have you asked anyone about the ownership?”