This IS the soundcheck of Oxford '73. It is available in almost VG+ quality. The one I had was one big track, more then 20 minutes, I believe. They played some very interesting things, like Ogre Battle without guitar.
There are some nice comments during that soundcheck, you mentioned already one, P_G.
I didn't download this file, but the comment of 'a wet fart' did ring a bell.
After all, very interesting and this is certainly NOT a studio-version (In my mind, they never recorded a studio version of this song, but hell, who knows?).
John: "It's the one thing I wish I could do - sing."