Solved! We rang travel agent to cancel and they were equally shocked.Took a while to get an answer but NCL have today rung to apologise for the stance wrongly taken by their staff. Electric wheelchairs, with certain provisos, are allowed.
NCL booking terms and conditions are clear that mobility problems must be delared in writing at the time of booking - if you didn't do this then I fear you are at their mercy. That being said I have travelled with many major cruise lines (not NCL) and have noticed loads of electric chairs, however using the gangway is problematic so best to use a standard chair then and on any excusrions.