Have you checked the time line associated with the sale?
I read on here that some people had tried and failed all over the christmas and new year period to contact sedo.
Maybe the buyer couldnt reach anyone and wanted the domain so much, they got peeved and a few weeks later when sedo were back they had gone off the boil.
You should put it back up for auction and any less than 6k, pay ya very small fee online and take it to small claims, you can claim upto 5k there, I'm sure that will cover the shortfall should you sell for less.
Make it rock solid like it sounds, and the magistrate will surely rule in your favour, after all you had spent that money and were relying on it thinking the sale was a done deal.
If they do not turn up the judgement will go for you, if they dont pay, simply have the judgement tranferred to the sheriffs court in london and they will rule in your favour. You can then send any bailiff to their address and in or not, they can gain access and remove enough to meet your debt.
So sedo lose 25k worth of stuff to raise your 5 k at auction, seeing as auction items from bailiffs typically get around 20% of the value.
As my kids would say, do it..do it...do it lol
I'm sure sedo will sort it for you, but why not make an article out of it and ask for it to be published in pauls new domain magazine?