Every killing affected her like a drug. The racing heartbeat. The soaring metabolism. The rising blood pressure. The surge of adrenalin.

Afterwards she always felt the need to relax and recharge her batteries. And there was no better way to do that than by letting her hair down and having a good time.

It usually included a fair amount of alcohol – her favourite drinks being tequila cocktails and champagne. But this assignment wasn’t over so she had to keep a clear head.

She was now sitting in front of the iPad, waiting for it to power up, while nibbling on a pre-packaged ham sandwich.

She was delighted with the way things were going. Two hits in two days and she hadn’t encountered a single problem.

Marion Nash had been an easy target. Just as easy as Dave Prentiss had been. Rosa’s mind carried her back through the sequence of events. Everything had gone according to plan.

When she’d left the hotel room it had been almost dark outside. On the way to pick up the bike from the garage she’d popped into a charity shop where she’d paid a few pounds for a man’s heavy black overcoat. It was two sizes too big but perfect for what she’d wanted it for. She’d sat on it during the short ride to Clapham.

The tiny independent bookshop run by Mrs Nash was right on the High Street. Rosa had parked the bike in a quiet street nearby that was not covered by any CCTV cameras. There she’d slipped the coat over her leather suit, put the helmet in one of the saddlebags, and replaced it with her own baseball cap.

The evening was cold and damp and everyone she passed had been in a hurry to get where they were going. No one paid any attention to the figure in the long coat and cap who could have been a man or a woman.

She’d stood watching the shop for the best part of twenty minutes from a doorway across the street. Through the window she’d been able to see Mrs Nash, and it was clear that the woman was working alone.

Only one customer entered the shop during that time: a young man in a raincoat who only spent a few minutes in there before leaving without buying a book.

Seconds after he’d gone Rosa had made her move. Head down, overcoat collar pulled up, she’d crossed the road between the slow-moving traffic.

A bell had rung as she’d entered the shop. Mrs Nash, a tall, middle-aged woman with dyed blonde hair, had been standing behind the counter.

Rosa had lifted her head just far enough to scan the interior and spot the single surveillance camera on the far wall. Having located its position she avoided looking at it as she moved between head-high bookshelves towards the counter.