Deep pleats are achieved by cutting a thin channel in the sew foam. You need to make the width of the material wider to allow extra material to flow into and out of the channel. If you are using 1 inch foam and the pleat is 1 1/2 inches wide, the material would need to be marked at least 3 inches wide. It would be best to cut a strip of material and lay it in the channel to determine an exact measurement.

To mark the foam, lay out your pleats and use a sharpie to draw the lines on the foam. With a new razor blade cut along the outside of the line on both sides of the mark to give you the thin channel. Be careful that you do not cut all the way through the foam backing. Remove the thin piece of foam.

Start sewing from the center of the insert and work your way to the outside. Chalk the sew line and use it as a guide to keep the stitch straight.

A narrow presser foot will also help when sewing.

merc flutes (oem style) can be done 2 ways...

sew the pleat lines straight onto leather/vinyl with calico on the back, mark a line up the centre of each pleat on the calico, move the face material out the way and run a fold on the calico down the centre and machine 4,5,6,7,8 etc mm out of the calico...this will give the pocket then stuff with a square edged firm stuffing.

or mark the calico and sew directly to the sew lines, rather than folding and sewing the calico.

the ends of the leather need to be pleated to get the leather to the size of the base material as you have say 40cm width in the calico but the leather on top made into a pocket is 50cm, is is done in a Z pattern.

that's with a twin stitch but same principle... you won't get a deep square edged flute, butted together with out making a pocket.

I've done them the first of Gareths ways and it works well.Also I've come across mercs where the foam is longer than the material but comes out through a cut in the calico and isn't sewn through on the piping edge but hangs below.
Any idea Gareth on whats the best option for foam to stuff them with thats available in th uk?

I’ve used jute, that was what was in there originally

You have directions in the book Russ, we call it channeling. These guys show good examples.