This was an Epic game (one of many) which was happening at GamesDay 2001.
Those are Ork Gunwagons advancing on some Imperial Guard Leman Russ and Hellhound tanks.
Back then there were quite a few games of Epic on show at GamesDay, this year there was (as far as I could see) just the one.
I have quite a large miniatures gallery of Epic miniatures on my website.