Try signing up for emails on the discussion email list on the westerly owners group - http://www.westerly-owners.co.uk.
If you can make it through the yahoo registration, posting an email to the list should get a response.
You could also try the contact links at http://www.westerly.co.uk, although this site doesnt seem to be updated that much.
Trafalgar Yachts specialise in Westerly stuff, and may have the info you need.
When getting quotes for my Westerly Merlin, the sailmakers seemed to have access to the measurements, but they insisted on measurements to back them up as individual boats can differ.