For the record I tried flattening the vinyl based tiles under books. This did sort of work, but they still resorted to a curl after a period of non compression. I also experimented with gluing pine wood strips to the edges. Both White glue and hot glue bonds were too weak (white glue didn't stick at all) so I abandoned that idea.
After a bit of head scratching and beard stroking I decided to trial mounting the printed boards on mounting board, quite often used for displaying artwork or presentations. Made of cardboard, not to be confused with foam core.
This has left the boards pushing together nice and flat, as you can see in the picture above. I'm on Holiday for a few days soon, so I'll leave them just "in box" and see if they are still OK on my return (fingers crossed).
As an added note, these alterations have been made using Dave Graffam's alternative format of selling the Photoshop files of several of his designs. The two above are from the Small Shop and Rustic Cabins #2 sets. Photoshop is a bit pricey, but I use the free software GIMP which handles all the graphics and layers in the original format just fine. This has the added bonus of being able to apply the different layers as per the normal PDFs, but also make my extra cut sections, change the location of elements (window positions for example) and make new image files so that the scaled down versions can be entirely printed on a single sheet of card, saving time and money. I would personally recommend them over the pdfs where available (especially as they are the same price!)