Hello,
I think that Thorn are going the way of Rohloff by just supplying those type of bikes other than Sherpa, Club Tour and Mk 3 Audax. In my own opinion not the best way to go, but its obviously working for Thorn. It is possible to get XTC etc previously used on the open market and also I think that Thorn would build a frame for you but at a greater expense.
Its the way things go.
I hope you find one somewhere as its a real shame longing and then not finding!
In fact the St John Street Cycles website lists Thorn XTC frameset in stock in four sizes, so maybe one of them is your size?
John.