As I recall, the early bird, which included the extender, was £30 (non-early bird £35). I selected the early bird and got the £30 pledge but it explicitly didn't list the extender board. With the extender it was £35, hence my original post.
On trying now, there is no early bird so the base + extender is shown as £35. It's £8 for shipping, so that's £43 for the minimal system to play around with, and that's too much for what might turn out to be shelf-ware, sorry.