I have a 56 plate Zafira and experienced the exact same problem. The boot lock and window washers run on the same fuse. In the extreme weather conditions the UK is experiencing, the washer mechanism can freeze up, either in dispensor, tubing or nozzle(s), which can blow the fuse. So if your window washers aren't working also, I'd suggest checking/replacing the fuse. I did this and presto-the boot lid opened on the first attempt!
Mrs. M Jack, January 2010