Can't seem to get AngularFire '$update' working - angular-ui-bootstrap

Question

I’m pretty new to Angular, Firebase and AngularFire, so it's probable I'm going about this the wrong way.
Basically I have a form in a modal(UI Bootstrap) and I want to update some previously stored values, but AngularFire ‘$update’ doesn’t update them in Firebase. Creating and deleting items outside the modal is working fine.
This is within my service:
updateItem: function (id, item) {
var item_ref = new Firebase(FIREBASE_URL + ‘/items/‘ + user_id + '/' + id);
var item = $firebase(item_ref);
item.$update({
name: item.name,
notes: item.notes
});
}
This is within my controller:
$scope.edit = function(id) {
$modal.open({
templateUrl: 'views/item.html',
backdrop: 'static',
keyboard: false,
resolve: {
data: function() {
return {
title: 'Edit item',
item: Items.getItem(id)
};
}
},
controller: 'EditItemCtrl'
})
.result.then(function(item) {
Items.updateItem(item.$id, item);
});
};
This is my modal controller:
app.controller('EditItemCtrl', function ($scope, $modalInstance, data) {
$scope.data = data;
$scope.ok = function(item) {
$modalInstance.close(item);
};
$scope.cancel = function() {
$modalInstance.dismiss();
};
});
This is my modal template:
<div class="modal-content">
<div class="modal-header">
<button class="close" style="margin-top: -10px;" type="button" ng-click="cancel()">×</button>
<h3>{{data.title}}</h3>
</div>
<div class="modal-body">
<form name="editItem" role="form" novalidate>
<div class="form-group">
<label class="sr-only" for="itemName">Item name</label>
<input name="name" type="text" class="form-control" placeholder="Item name" value="{{data.item.name}}" ng-model="data.item.name">
</div>
<div class="form-group">
<label class="sr-only" for="itemNotes">Item notes</label>
<textarea name="notes" class="form-control" rows="2" id="itemNotes" placeholder="Notes" ng-model="data.item.notes" ng-maxlength="500">{{data.item.notes}}</textarea>
</div>
</form>
</div>
<div class="modal-footer">
<button class="btn btn-default" type="button" ng-click="cancel()">Cancel</button>
<button class="btn btn-primary" type="button" ng-click="ok(editItem.data.item)">OK</button>
</div>
</div>
Stepping through everything shows that the item to be updated and new values are being passed through to the service, but they're not making their way into Firebase.
I'm guessing that I'm probably going about this the wrong way though - any guidance would be much appreciated.