They connected the battery up the wrong way round?? Has it run at all since that happened? I'm not an electronics man but I know that the sort of damage that CAN cause is exepensive. Depending on the wiring of the Micra it could easily have blown all the fuses, it could have fried your ECU I guess, it could have done a bunch of really not nice things.
I would be straight down to Halfrauds shouting the odds. Like Arnold menitoned, can you jump start it?
