A fellow Englishman of yours on a different MB forum had unfortunate experience of replacing his 722.6 gearbox on his 9? SL600 and dealt with various shops such as Automatics in West London. Mercstar in Harrow, etc.
He appears to be very knowledgeable about the R129 and its internals. His forum name is bluesl and the link to thread detailing his experience is -
http://www.mbworld.org/forums/showthread.php?t=71930

Good luck!

97 SL500 tri blk
sprt pkg 1
28k miles