You do know the new Vixen software / firmware update for the Starbook from April this year completely fixes the declination jump bug Richard? The SXW I have now tracks and guides as well as the GPD2 (which is incredible).
If you don't patch the software, Vixen also now tells you how to avoid the dec jump issue entirely (last alignment is done MANUALLY instead of syncing with the slewing). Easier just to patch it