PS; if it's any help the CAP413 reference is at 1.4.2:
1.4.2 All numbers, except those contained in paragraph 1.4.2 b) shall be transmitted by
pronouncing each digit separately as follows:
a) When transmitting messages containing aircraft callsigns, altimeter settings, flight
levels (with the exception of FL100, 200, 300 etc. which are expressed as ‘Flight
Level (number) HUN DRED’), headings, wind speeds/directions, pressure settings,
airspeed, transponder codes and frequencies, each digit shall be transmitted
separately; examples of this convention are as follows:
[...]
b) All numbers used in the transmission of altitude, height, cloud height, visibility and
runway visual range information which contain whole hundreds and whole
thousands shall be transmitted by pronouncing each digit in the number of
hundreds or thousands followed by the word HUNDRED or TOUSAND as
appropriate. Combinations of thousands and whole hundreds shall be transmitted
by pronouncing each digit in the number of thousands followed by the word
TOUSAND and the number of hundreds followed by the word HUNDRED;
examples of this convention are as follows: