I experienced this with my original TBA, as well as 1 or 2 others, when we were out riding and got cuaght in the rain. The bike loses power and then stalls. Just try loosening the gas (petrol) cap a little to break the vacuum. She should come right back.
Soren is right, some people have cut their breather tubes at an angle to get more air flow in. Something I will have to do on the "new" TBA, maybe when I replace the in-line "secret" filter.

Blue/White 2007 TBA, Thruxton needles, Unifilter, AI removed, Polaris Bellmouth, Bubs, Nology Coils/wires, Lightbar, Ricor Intiminators, Hagon Nitros, Tall Sissy Bar w/luggage rack, Dart flyscreen & Lowers. 130 Mains, TrueGel Battery MG12-BS.