Thanks, that worked great (although it's ClrFlag(SelectedLines,join)) james
If your previously laid-down tracks didn't have the clear flag set, you can at least edit the .pcb file and add the "clearpoly" flag to all the lines.
You can use the J key to selectively un-join lines, or Shift-J for selected lines, but as it's a toggle, you can't just select *all* and apply it, as it will re-join previously unjoined lines.
You can select all and do :SetFlag(SelectedLines,join)