Book 5: Reunion, the conclusion to the story begun in Book 4: Unsafe Haven will be out early in 2014. It currently stands at 23,000 words, beginning just outside of Penrith a few months before the end of Book 4, then following the journey south. The story then switches point of view, and follows a journey to Hull.
The above will make more sense when you've read Book 4. So will my choice of a working title for Book 6, I've been calling the draft document Svalbard.
I've a title for the US spin off, but I'm keeping that one under my hat for now.