The method turned out to be reasonably simple.
Here is a photo of most of the stages to produce the effect.
From left to right:
1. Slaters Brick Sheet - This terracotta coloured plastic sheet comes in English Bond, Flemish Bond and Stretcher Bond.
Not included in the photo (no example piece of brick sheet) - Brick sheet lightly sprayed with Tamiya XF57 Buff acrylic paint. It is best to allow some of the terracotta to still show through the buff in random areas, don't spray evenly all over, thicker and thinner randomly.
2. Brick sheet showing mortar lines - A wash of Jo Sonja Soft White acrylic paint was used to enhance the mortar lines and to shift the brick colour of the buff painted bricks. A blotchy application is not a problem as it assists with the uneven colouration of the common bricks. Any off white paint suitably thinned to give a similar effect could be used. Jo Sonja acrylic paints are available in craft and art supply stores. I find them very useful for backscenes and model painting as they have very fine pigment and cover well unlike a lot of current model paints.
3. Brick sheet with a heavy wash of the off white - The random darker colour of the common bricks has been produced by a brown water colour pencil applied in varying dashed and short stokes both horizontal and at angles. The bricks on the upper floor of the Werris Creek station building are English Bond which has a row of end bricks then a row of the normal long side bricks then it repeats (see photo above).
4. Brick Sheet with a light wash of off white - The random dark parts of the common bricks are reproduced the same was as in 3 above.
I cut enough English Bond sheets for the upper floor and applied the off white wash then picked sheets that matched for each long side of the station. The join will be covered by a downpipe from the roof.
On the actual station the platform level and the upper level were built at different times and are in two bonds. The platform level is in reddish terracotta brick Flemish Bond and the upper level is in common brick English Bond.
The terracotta brick with white pointing (specially treated mortar lines) of the platform was achieved by using the off white wash for the mortar which turned the bricks pink. The bricks were then brought back to the terracotta colour by scrapping with the straight edge of a single sided razor blade held vertically to the brick surface, very easy.
This photo shows the final terracotta Flemish Bond platform level wall.