Thanks for the email Midland Martian- heres the texts of 2.18 and 2.19 from the 1991 edition:-
2.18 Every dwelling should have access to alternative escape routes so that a person confronted by the effects of an outbreak of fire in another dwelling can turn away from it and make a safe escape.
However, a single escape route from the dwelling entrance door is acceptable if either:
a. the dwelling is situated in a storey served by a single common stair and:-
i. every dwelling is separated from the common stair by a protected lobby or common corridor (see Diagream 10), and
ii the travel distance limitations in Table 2, on escape in one direction only, are observed; or
b. alternatively the dwelling is situated in a dead end part of a common corridor served by two (or more) common stairs, and the distance to the nearest common stair complies with the limitations in Table 2 on escape in one direction only (see Diagram 11).
Small single-stair buildings
2.19 The provisions in paragraph 2.18 may be modified and a single stair, protected in accordance with Diagram 12, may be used provided that:
a.the top floor of the building is no more than 11 m above ground level, and
b.there are no more than 3 storeys above the ground level storey, and
c.the stair does not connect to a covered car park, except if the car park is open-sided.
Interesting that despite this the Department determined that the stay put policy was ok...not sure I would have said that!