Takedown (Lex Harper #1) by Stephen Leather

When a British Special Forces soldier goes rogue, carrying out an attack at a US army base in Syria leading to the death of two men, ex-MI5 controller Charlotte Button is hired to work out what his plan is and to take him down before he can carry it out.

Charlotte puts her best man on the case – hitman Lex Harper. It’s up to Lex to assemble a crack team and get to the soldier before he carries out what could be a massive terrorist attack. But Charlotte might require Lex’s help with a more personal problem too . . .

As ex-MI5, Charlotte holds information on thousands of the agency’s dirty operations, and keeps a store of those secrets in three flash drives hidden in separate safety deposit vaults. Now someone is taking extraordinary measures to recover the secrets through Hatton Garden-style robberies. So far two vaults have been raided, and two flash drives have gone. If the third is stolen, Charlotte’s life will be in jeopardy. While it’s down to Lex to prevent a potential terrorist attack on British soil, Charlotte must work out who is after her and what they want.