William Mackay Luthiery is a member of the British violin makers Association and can be found listed as a bass maker on the following site makers directories.

British violin makers association emblem.
SUPPORTING THE CRAFTS OF VIOLIN AND BOW MAKING IN THE UK & WORLDWIDE
The British Violin Making Association, formed in 1995, is one of the foremost violin making associations in the world, with about 500 members based both in the UK and abroad.
 
http://www.mooseweb.co.uk/bvma/williammackay.html
Gollihur Music.
The Gollihur bass luthier directory.  The Gollihur music website is also a great resource for all things bass.
https://www.gollihurmusic.com/luthiers.cfm
William Mackay Luthiery is a Patron of Thomas Martin, Martin Lawrence & George Martins book.
   
The English Double Bass.
A fascinating new book that explores the history of the double bass in England and the impact Domenica Dragonetti and others had on its evolution.
And also the next publication from arpeggio
The Italian Double Bass.
www.arpeggio-publishing.com