The Coincidence Engine

Winnie Fettle cannot stop winning.
Every raffle. Every tombola. Eleven coin-tosses in a row, until the whole pub went quiet. And her nan always said that luck like that comes with a bill.
Tabitha and Barnaby Quill have just inherited a detective agency above an umbrella shop in a foggy canal town, left to them by a great-uncle who did not die - which leaves paperwork - but vanished, which leaves none. Winnie is their first paying client.
Tab is thirteen. She keeps a private ledger of what everybody in town is worth to her, and she does not believe in the kindness of buses.
Barney is eleven. He simply knows things, and can never once say how.
Beneath their new town runs the Serendipity Service: a hidden civil service that keeps the world's coincidences in order, one form in triplicate at a time. Something down there is rigging chance itself. And somebody grieving, brilliant and entirely reasonable means to use it to abolish accidents forever.
Tab plans. Barney knows. Neither of them can crack this one alone.
Every clue is on the page. See if you can beat them to it.