The original question was quite specific and wanted a simple English explanation. Please let me attempt this sorry where I repeat what others have said. And i have tried to answer some other questions in this thread.
In sort the thing you have quoted means you have to have both background and purge ventilation in the kitchen and the conservatory.
The back ground ventilation is a fixed quanity it is 8000mm2 (80cm2) regardless of the size of rooms.
And it must be above head height - at least above my head - at 1.7 m.
The purge vent must be calculated based on the size of the kitchen and conservatory together.
It does mean that the purge vent in conservatory must be to outside.
My source of reference is building Regs Part F so please do check up!
http://www.lbhf.gov.uk/Images/Part_F_2006_tcm21-19841.pdf
Apdx B
Purge ventilation means windows and door that open.
For measurement purposes APDX B says in sash windows the 1/20th has to be the measurement of the open space
1/20th of the opening part of a pivot window that pivots 30degrees or more
But if it pivots less than 30 degrees opening part needs to be 1/10th.
MATHS
You know how to work out the floor area. Eg 3x3 kitchen and 3x3 conservatory 18 square metre.
So 1/20th for purge is 18 / 20 = 0.9 m sq
Or 1/10th is 18 / 10 = 1.8 m squ
And it all means in this eg you would need a door or window with at least a 0.9m sq opening from kitchen to conservatory and the same from conservatory to outside. And that if purge ventilation was only just adaquate before the conservatory then it will not be enough after a conservatory is added.
You can have several windows and doors added together to get the right measurement for purge ventilation.
Back ground ventilation generally means trickle ventilation typical in windows, or air brick.
Second source of
Reference GSR TB 005(A) which replaced 005
"Gas cookers in Internal Kitchens"
This document states that if the kitchen is 10 cubic metres or more it simply needs have an openable window which complies with the Building Regulations rules above.
So you need to refer to TB005(A) further only if the kitchen is very tiny and/or does not have the ventilation required by current building regs.
What about a Kitchen with No Windows for Purge Ventilation? TB005 (A) states:
Intermittent extract fan above hob 30 l/s
Or
if Located elsewhere in kitchen 60 l/s
It can be manual or automatic.
15 minute over run facility