Hi,
Have you removed the 21A transmitter from it's housing as detailed in the following link?
http://www.britishtelephones.com/tranchan.htm
Apologies if you have already tried this.
Sometimes the THG group can be quite helpful with old phones and may even be able to source an original transmitter for you their site is
http://groups.google.com/group/thgdiscussion?hl=en-GB
but you have to become a member to post messages.
Regards
Andrew