How to use the Leaflet 'maxBounds' option?
Got it working. Override the leaflet.drupal.js file as follows. See the leaflet markercluster module for how to override in an 'As You Like it kind of way.' in a custom module. Pretty useful.     

// instantiate our new map
  var lMap = new L.Map(this.mapId, settings);
//note, the comma (NOT A semi-colon) at the end of the next line - this is  important.
  lMap.bounds = [],
// Now set your maxbounds, see the leaflet documentation for reference.
    lMap.setMaxBounds([
      [40.712, -74.227],
      [40.774, -74.125]
    ]);