Not sure about the tandem manufacturer being THORN, I cannot find what I remember seeing online at the moment, but I am fairly certain they do their frame building in house. Last time I went to SJS I had a look around their complex and there is certainly an extensive workshop.
I can't think who else it might be. Lee writes:
"in 1995 someone contacted us to build Tandem frames. This led to touring frames and audax frames. They have become one of the biggest suppliers of Tandems and Tourers worldwide. We built hundreds of frames for them until 2005 when they changed their focus and went to Taiwan"
The only other major player in touring and tandem frames in the UK is Dawes, and their own production moved from Birmingham to eastern Europe during the relevant period. Thorn did have a small production facility of their own, but when I was shown around their premises a few years ago, I was told that the majority of their frames were built in the Far East.
There was a recent case of a forum member trying to obtain a warranty repair on a cracked Thorn frame who was told they no longer have any framebuilding facilites. The repair would have been undertaken by a subcontractor.
Calmes dans le demi-jour
Que les branches hautes font,
Pénétrons bien notre amour
De ce silence profond.