It's a two part that used to be supplied by Westerly - I went down and took delivery of some at Waterlooville, but that was a while ago. Try their successors.
It was a lacquer made by Trimite of Uxbridge
You could post this on the Westerly Owners discussion list, you may find an owner who has sourced some recently. Westerly-Owners-subscribe@yahoogroups.com
Martin