A fellow guest said: "Emma had gone over to speak to Pixie Geldof, who was chatting to Johnny at the time – and Emma and Johnny immediately hit it off.
"It was clear they had loads to talk about – even if they don't look like they have much in common."
The couple shared a cab to another fashion party, thrown by designers PPQ at the Dolce nightclub. From there, they are believed to have gone on to a private event at the Dorchester.
But there was no sign of Miss Watson by 2.30am, by which time Borrell and another female friend were being shown the door for their over-exuberant behaviour.
Miss Watson, who is said to havemade a £10million fortune from the Potter film series, gained ten top-grade GCSEs and hopes to study philosophy at Cambridge.
But in recent months she has been keen to distance herself from her goody-two-shoes image.