From 4444:
8.8.1 Emergencies
8.8.1.1 In the event of an aircraft in, or appearing to be
in, any form of emergency, every assistance shall be provided
by the radar controller, and the procedures prescribed herein
may be varied according to the situation.
8.8.1.2 The progress of an aircraft in emergency shall be
monitored and (whenever possible) plotted on the radar display
until the aircraft passes out of radar coverage, and position
information shall be provided to all air traffic services units
which may be able to give assistance to the aircraft. Radar
transfer to adjacent radar sectors shall also be effected when
appropriate.
Note.— If the pilot of an aircraft encountering a state of
emergency has previously been directed by ATC to operate the
transponder on a specific code, that code will normally be
maintained unless, in special circumstances, the pilot has
decided or has been advised otherwise. Where ATC has not
requested a code to be set, the pilot will set the transponder to
Mode A Code 7700