There are two papers by Morwen Thistlethwaite on this subject, both dating
from 1980.
I got these copies from David Singmaster after having tried unsuccessfully
to get them from Morwen himself.
I have since then made many copies of these notes for many people all around
the world.
I will continue to do this as long as I feel comfortable about it.
So if there is still anybody out there who wants them, please e-mail me
directly at geohelm@pt.lu or at georges.helm@comnet.eo.lu
Georges Helm