After much experimenting and googling I came across the answer to this. The answer is as follows:
<key>UTExportedTypeDeclarations</key>
<array>
<dict>
<key>UTTypeConformsTo</key>
<array>
<string>public.data</string>
</array>
<key>UTTypeDescription</key>
<string>Writing Shed Data</string>
<key>UTTypeIconFiles</key>
<array>
<string>writingshed.png</string>
</array>
<key>UTTypeIdentifier</key>
<string>www.writing-shed.comuk.Writing-Shed.data</string>
<key>UTTypeTagSpecification</key>
<dict>
<key>public.filename-extension</key>
<array>
<string>wsd</string>
</array>
</dict>
</dict>
</array>
<key>UTImportedTypeDeclarations</key>
<array>
<dict>
<key>UTTypeConformsTo</key>
<array>
<string>public.data</string>
</array>
<key>UTTypeDescription</key>
<string>Writing Shed Data</string>
<key>UTTypeIconFiles</key>
<array>
<string>writingshed.png</string>
</array>
<key>UTTypeIdentifier</key>
<string>www.writing-shed.comuk.Writing-Shed.data</string>
<key>UTTypeTagSpecification</key>
<dict>
<key>public.filename-extension</key>
<array>
<string>wsd</string>
</array>
</dict>
</dict>
</array>