you can get the table ID from the following:
GET /documents/<reportID>/reports/<reportId>/elements.
the Response should contain a <VTable>. you can get its id from there:
ie:
<element type="VTable">
<id>13</id>
<reference>1.D</reference>
<name>Block 1</name>
<parentId>24</parentId>
...
Once you have the <id> rather than the reference, try the export.
Dan