‘Of course!’ Becks said, stepping back and looking at the other paintings hanging on the wall. ‘All the pictures in the room have frames, but this is the only one that doesn’t.’

‘Bingo!’ Judith said.

Becks leant in closer to look at the edges of the frameless painting.

‘And if you look closely,’ she said, like a doctor describing a particularly fascinating X-ray, ‘you can see how the wall is a darker colour around the painting. Where a frame has been stopping any sunlight from bleaching the paint on the wall.’

‘Okay that’s thorough,’ DS Malik said.

‘So this picture’s lost its frame very recently,’ Judith said, thinking the chain of events through for all of them. ‘Which explains the chisel, hammer and wood shavings on the desk over there. Don’t you think? The intruder broke in and was removing the frame from the painting when I interrupted him.’

‘Was he holding anything like a picture frame in his hand when he attacked you?’ DS Malik asked.

‘I couldn’t see,’ Judith said, frustrated. ‘It was too dark.’

‘Or maybe you noticed something in his hands as he fled?’

‘I’m sorry, I’d dropped to the floor by then. I didn’t see anything in his hands.’

‘But it makes no sense!’ Becks blurted. ‘The art in this room must be worth hundreds of thousands of pounds. Maybe millions. Who’d break in, ignore everything else and instead make off with a picture frame?’

Judith peered at the painting, and at the space where the frame had been. She then took a step back and looked at all of the paintings in the room.

‘It’s like a crossword clue,’ she said.

‘I’m sorry?’ Becks said, frowning.

‘A crossword clue never makes any sense when you read the surface meaning, but that’s because you haven’t decoded it yet.’

‘I don’t follow,’ DS Malik said.

‘Each cryptic crossword clue is made up of two parts. Generally. One half of the clue is the wordplay, and the other half is the basic definition. Unless it’s the sort of clue where the whole thing is both the wordplay and the definition. But let’s not get bogged down. The point being, when you know how to decode the clue, it makes perfect sense. But until that point, the clue appears nonsensical. Take “Two girls, one on each knee”, seven letters.’

‘Take what?’ Becks asked, struggling to keep up with Judith’s logic.

‘“Two girls, one on each knee”.’

‘Why?’

‘Not literally, just take the clue.’

‘What clue?’

‘“Two girls, one on each knee”.’

‘I have to be honest, I’m not really following this.’

‘It couldn’t be any simpler! “Two girls, one on each knee” is about the most famous crossword clue there’s ever been. Compiled by a splendid setter called Roger Squires. It was his two millionth clue. Imagine that!’