Dolly says that she had weighed herself and the scales were "hovering over the 7", but after she "payed a visit" they had dropped back down to the 6.
For Dolly to have lost 1 stone through urination her bladder would have had to have held approximately 11 pints (6.3 litres) of liquid.
There is no such thing as a "plunge teapot", what Dolly bought was a cafetiere (a coffee pot).