Are there any XSD experts out there who could help me?

We have an existing schema which I need to be able to extend. I need to add
extra choices to one of the elements.
I have created a new schema and am trying to redefine one of the elements to
add another element to the  choices.
However, I keep getting the following errors when trying to validate the
schema:

1) The base attribute of a restriction/extension must be the same as the
complexType's name attribute in <redefine>
2) Global complexType:'link' declared more than once or also declared as
simpleType

The link element is a complex type in the original schema.

The extended schema is below:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
elementFormDefault="qualified" targetNamespace="http://www.blah.com/data/">

 <xs:redefine schemaLocation="MainSchema.xsd">
  <xs:complexType name="link">
   <xs:choice>
    <xs:extension base="link">
     <xs:choice minOccurs="0" maxOccurs="1">
      <xs:element ref="expenses"/>
     </xs:choice>
    </xs:extension>
   </xs:choice>
  </xs:complexType>
 </xs:redefine>

 <xs:element name="expenses">
  <xs:complexType>
   <xs:sequence maxOccurs="unbounded">
    <xs:element maxOccurs="unbounded" name="expense"/>
   </xs:sequence>
  </xs:complexType>
 </xs:element>

 <xs:element name="expense">
  <xs:complexType>
   <xs:attribute name="amount" use="required" type="xs:decimal"/>
   <xs:attribute name="rate" use="required" type="xs:decimal"/>
   <xs:attribute name="type" use="required" type="xs:NCName"/>
  </xs:complexType>
 </xs:element>
</xs:schema>