Down/Up the swannee

It appears that down the swannee or up the swannee variously spelt 'swanee', 'swannie', or 'swanny', sometimes with one 'n' sometimes with two, are British colloquial expressions for failure or bankruptcy. According Eric Partridge and Paul Beale, these expressions have nothing to do with Swannee as in the river that flows from Southern Georgia into Florida and thence to the Gulf of Mexico. Unfortunately, Partridge and Beale offer no further etymological pointers other than that these expressions “may have something to do with swan” as in to “swan aimlessly around”. Partridge and Beale date 'down or up the swannee' from post 1950, which would suggest no connection to being up or down the Swannee River. The Stephen Foster song The Old Folks at Home (better known as Way down upon the Swannee River) written in 1851 did become very popular in Britain and, as some maintain, is the true source of these expressions. The citation evidence, however, is sadly lacking. Therefore the origin of these expressions remains an unresolved etymological debate.