As is usual in these cases, there is an official publication in which a definition can be found. The easiest one to access in this situation is probably LASORS (which can be downloaded from the CAA website free of charge).
LASORS (which is invaluable in any event) addresses this issue in Section A, Appendix B. An extract is copied below:
Aeroplane flight time
An aeroplane shall be deemed to be in flight from the
moment the aircraft moves under its own power for the
purpose of taking off until the moment it comes to rest at
the end of the flight.
Helicopter flight time
The total time from the moment a helicopters rotor blades
start turning until the moment the helicopter finally comes
to rest at the end of the flight and the rotor blades are
stopped.
Most other matters are discussed in this Section of the publication.
Additionally, if a pilot carries out a series of flights on the same day (such as glider towing or parachute dropping) returning to the same place of departure and the interval between successive flights does not exceed 30 minutes, such a series of flights may be recorded as a single entry.
AJ