For anyone who suddenly cannot open their boot (by the normal method of pulling up on the lid, and thereby pressing the activator button), this one's for you... I've read a lot to try to troubleshoot my (particular) problem. There is a 7.5a fuse in the engine bay fuse box (labelled as F11 "Tailgate Release") that had gone bad. I pulled out the "spare" (listed as F52 "Spare") and plugged it into the F11 spot. Now my boot opens again!! This would be a quick thing to try before looking at the actual switch itself and related wiring and relays, etc. I've also learned that you can manually open the "J"'s boot lid the same way as the "H" - there is a little plastic grommet directly above the boot locking mechanism, that pops out (which is accessible from inside the boot, via the rear passenger seats), and inside is a square hole, into which a wide screwdriver (or something square?) can fit into. A counter clockwise turn pops the boot open.

Hope this helps someone!