Blocky artifacts are within the video stream as far as I can tell, because the same video stream played on a PC with high speed broadband also displays the same artifacts. While their flash site for the PC has a high quality none blocky stream. If only the mobile stream was the same as the PC version because the N900 is more than capable of playing it.

Here is a high quality stream for BBC parliament channel it's used for IPTV so is TV quality and the N900 can play it fine on a none overclocked system using only 75% CPU