I looked up "swampy" at urbandictionary.com but am not sure if any of these definitions fit.
I had heard the word before, but I never knew what it meant except that it was uncomplimentary. Yours is a likely explanation.
I don't think that sod comes from sodomite. I say this only from its general usage: it's a very mild phrase.
The 'swampy' thing comes from an environmental protest from the mid-90s. Some guy, calling himself 'Swampy', dug himself a tunnel in an area where a bypass was being constructed and held out for a month or so, delaying progress on the road. It was a very big news story in Britain. So any unkempt jackass hippy is generally called Swampy now :)
Andrew