Works okay on 32 bit but not on 64 bit - indeed, it now crashes with segmentation fault in 64 bit. Valgrind log attached. Looks like json or potentially geant4. I'll try to get Valgrind working on my 64 bit machine so I can see what's going on there. (Nb: python is not compatible with valgrind - has it's own malloc system. Assume the native C++ stuff is not affected by this, but the python stuff is).

Humm, think it must have been some problem with my dev environment. Works fine in my trunk copy and no memory problems registered...