OK, further to my query above, it appears that if you are careful, the mast does not need stabilising lines, as long as the gin pole/spinnaker pole is tied down well through the jib sheet sliders. As I discovered the hard way though, you do have to be really careful that the pole is centered and the lines are tight, but it does work, as the rest of you will already know, no doubt.
Any hints and tips are gratefully received though.