Hi mor, that's a good news for you.
Personally, I use the decoder from www.svideo.com. Simple and neat.
Here are my ideas about controlling the ATV but I am not expert with BV5. This is in the case the ATV is connected to HDR1 (not directly to TV).
If it is the same as the BV7, you can control the ATV by running an IR blaster from the BV5 to the ATV. In the connection menu of the BV5, you select V.MEM which will give you another option to select the equipment connected to your HDR1. In this case, you should update the chip (to at least february 2008 version) of the BV5. This operation cost around 100-300Euro and must be done by B&O service. The pro of this solution is the direct axess to ATV with Beo4. The cons are the update costs.
If BV5 is like the Avant (no possibility to add a second level equipment), in your setup, you must connect the IR blaster from HDR1 to ATV. In this case you should update the software of HDR1 through the service tools available at your B&O dealership. This operation is usally free if you are a good customer... The drawback of such solution is that you must control the ATV through HDR1 menus. The pro is the low cheap cost of the upgrade.
In case you connect ATV directly to TV (like I did), the IR blaster goes from TV to ATV and you must update your TV like in the first solution. I still have to do the last part so, for now, I control the ATV using the AppleRemote.
I hope this is useful. Cheers.