I am sure you already know or someone has mentioned that the Lakes Gliding Club here in England at http://www.lakesgc.co.uk/
has the complete book on Slingsby Sailplanes with drawings and pics etc,. Go to http://www.lakesgc.co.uk/
then click on 'Archieve' then 'Old Gliding Books' then 'Simon M Slingsby Sailplanes' and on page 151 he goes into the T31 history with drawings and pics etc,.
(You sure have a super country in NZ,I can't stay away when it's your summer).