In their minds, she was already a Missing Person, what police called a ‘MisPer’, but they’d waited till next morning before calling Flip’s mother at the family home in East Lothian. Mrs Balfour had wasted no time, dialling 999 immediately. After receiving what she felt was short shrift from the police switchboard, she’d called her husband at his London office. John Balfour was the senior partner in a private bank, and if the Chief Constable of Lothian and Borders Police wasn’t a client, someone certainly was: within an hour, officers were on the case – orders from the Big House, meaning Force HQ in Fettes Avenue.

David Costello had unlocked the flat for the two CID men. Within, they found no signs of a disturbance, no clues as to Philippa Balfour’s whereabouts, fate, or state of mind. It was a tidy flat: stripped floors, fresh paint on the walls. (The decorator was being interviewed, too.) The drawing room was large, with twin windows rising from floor level. There were two bedrooms, one turned into a study. The designer kitchen was smaller than the pine-panelled bathroom. There was a lot of David Costello’s stuff in the bedroom. Someone had piled his clothes on a chair, then placed some books and CDs on top, crowning the structure with a wash-bag.

When asked, Costello could only assume it was Flip’s work. His words: ‘We’d had a falling-out. This was probably her way of dealing with it.’ Yes, they’d had arguments before, but no, she’d never piled up all his stuff, not that he could remember.

John Balfour had travelled to Scotland by private jet – loaned him by an understanding client – and was at the New Town flat almost before the police.

‘Well?’ had been his first question. Costello himself offered an answer: ‘I’m sorry.’

Much had been read into those words by CID officers, discussing the case in private. An argument with your girlfriend turns nasty; next you know, she’s dead; you hide the body but, confronted by her father, innate breeding takes over and you blurt out a semi-confession.

I’m sorry.

So many ways to read those two short words. Sorry we argued; sorry you’ve been troubled; sorry this has happened; sorry I didn’t look after her; sorry for what I’ve done …

And now David Costello’s parents were in town, too. They’d taken two rooms at one of the best hotels. They lived on the outskirts of Dublin. The father, Thomas, was described as ‘independently wealthy’, while the mother, Theresa, worked as an interior designer.

Two rooms: there’d been some discussion back at St Leonard’s as to why they’d need two rooms. But then, when David was their only son, why did they bother to live in an eight-bedroom house?