With the advent of the excellent Dapol Southern Railway Maunsell coaches, we’ll need to know how to put together a realistic formation.  I came across this useful information on the Worsley Works etched ‘scratch-aid’ and kit site (originally from the Southern e-group, I think):

The Southern Railway built many of its coaches to run in sets as follows:-

Five Coach Sets
Semi-Open Brake Third
Corridor Third
Corridor Composite
Corridor Third
Semi-Open Brake Third

Four Coach Sets
Semi-Open Brake Third
Corridor Composite
Corridor Third
Semi-Open Brake Third

Three Coach Sets
Semi-Open Brake Third
Corridor Composite
Semi-Open Brake Third

Maunsell Coaches ran in the same formations as above with a Corridor Brake Third replacing the Semi-Open Brake Third.  The following additional formations were also used:

Six Coach Sets
Corridor Brake Compo or Brake Third
Corridor Third
Corridor Composite
Corridor Composite
Corridor Third
Corridor Brake Compo or Brake Third

Five Coach Sets
Corridor Brake Compo
Corridor Third
Corridor Composite
Corridor Third
Corridor Brake Compo

Four Coach Sets
Corridor Brake Third
Corridor Composite
Corridor Composite
Corridor Brake Third

Two Coach Sets (Introduced 1959)
Corridor Brake Compo
Open Second

The Corridor Brake Composite used in the two coach sets was designed for loose working with as many as seven included in the Atlantic Coast Express.

No excuses for incorrect formations now!