Incidentally, I found another couple of interesting links mentioning BTH and the history of the company (bottom one is best, although the first shows an interesting speaker from the 1930s):

http://www.radiomuseum.org/m/bth_gb_en_1.html

http://www.warwickshireias.org/pdf%20docs/aeirugby.pdf

They are certainly a company with a respectable pedigree!

Enjoy

Marco.