The process to build a section of flat wall is as follows.
1.Take a photo of the wall square on, well lit.
2. Count the bricks and make a scale drawing and then a master, building in all the detail you like, curved arches, denticing, rebates, anything goes. I built my master out of 3 mm ply and covered it in plasticard, Ratio or Wills sheets and sections, arches, oriole windows, whatever. You have to think where the joints are going to be and where the repeat line falls. I decided to conceal my joints behind buttresses on the outside. For the inside panels, I added plasticard overlays.
3. I made a mould tray out of ply and lego bricks about 5 mm deeper than the master and about 8 mm larger all round. The bottom layer of Lego bricks has to be stuck down.
4. The master is stuck down to the bottom of the mould tray, right way up, taking care that there are no air spaces under it. I used double sided sticky tape.
5. Mix the latex and pour a little into the mould tray, lightly brushing the mix into the master so that all air bubbles are excluded from the detail. Fill up the mould tray to the required depth. You want a strong mould so do not skimp it.
6. Leave to set for four days.
7. Carefully dismantle the lego walls of the mould and release it from the master. Detach the master from the base of the tray. Rebuild the lego walls of the tray, invert the mould and place it in the tray where it will be a snug fit. Carefully trim away any flash with nail scissors.
8. Start casting the wall sections using special modelling plaster. Taranti's Alpha plaster is finely divided to pick up the detail and has high strength. You have to brush some plaster into the detail before filling the mould. I found that I could do two casts a day. I was casting an inside wall section, an outside and a buttress at the same time. I vibrated the bubbles out of the plaster on the top of my pillar drill casing, in which the belts rattle round, a perfect vibrating table. You have to keep the moulds flat in the trays or you get curved walls. I place thin strips of wood top and bottom and placed a weight over them. I use the heavy coils from defunct fluorescent lights. They are about 150 mm long and have a flat base; these are also good for holding down track whilst glueing.
9. A curved ply wall inner was built and then the wall sections were stuck to it using Contact adhesive. For a wall with windows make the wall inner out of perspex. The wall sections had first to be sanded to a constant thickness and the edges neatened to make a good fit. The buttresses were glued over the joins and the gaps were filled with white PVE glue from a hypodermic needle.
I have since built an engine shed using this method and I plan to build retaining walls for a long cutting based on Belper. This system is great for long repetitive walls. You can make your master as complicated as you like, you only have to do it once.