I have yet to see my amp detect the presence of 5.1 from my Humax Freesat. Is 5.1 actually broadcast on free sat and if so what channels? I am using the optical cable to connect it to my Denon AVR3803.

The BBC-HD preview loop is in 5.1 (The test card has a 5.1 level and speaker location test). The main Strictly Come Dancing show is 5.1 as is Jools Holland.

When I first got freesat HD (in May 2008 for the European cup) all the games the BBC covered were broadcast in DD5.1. Created a great sound with commentary from central speakers and crowd noise behind. Haven't noticed if they still do this. ITV HD never seem to.

My plasma does not support HD which is why I have not seen any 5.1 coming in.

Tv's don't normally support 5.1, to listen to Dolby Digital requires a Dolby Digital decoder like you get in a AV amplifier.

I know that I have an AVR3803, but as I don't have a TV that supports HD, I don't tune to the HD channel.

Not sure what an AVR4803 is. But two things spring to mind

1 the picture from the hd channel is likely to be way better than watching SD because of the extra bandwidth used for the HD channel

2 The 5.1 surround sound is only transmitted on the HD channels. If you watch in superior SD, from the HD channels, you still get surround sound ( if transmitted)t the same as as a dvd)

As my plasma can only receive SD why do you think that the SD output by down scaling HD channels is better than broadcast SD. I don't think I can see any difference between the two.

In theory, yes.

HD channels are coming down at a much higher bandwidth than SD channels, so in theory, the picture should be slightly better even when downscaled to the same resolution.

If you've ever explored a digital camera, most of them have a "fineness" setting, which determines how compressed the image is stored. Higher compression = smaller file = worse picture.