CAP 393 Section 1, part 8, page 2:
(5) For the purposes of this article, a helicopter is in flight from the moment the helicopter first moves under its own power for the purpose of taking off until the rotors are next stopped.
So strictly by the book, an engineer can legally start the aircraft from cold, but he can't take over "rotors running" then stop it after it's first moved under its own power....