Most DVD recorders (and video recorders) have a TV tuner built in. When you record something on a DVD (or Video) it is actually recording from the TV tuner inside the DVD or Video.
This is how you can watch one channel while recording another. You watch the TV while recording from the TV tuner in the DVD or Video.
But the TV tuner inside your DVD (or Video) recorder is an probably an analogue tuner so does not know about channels like Discovery etc.
So you need to feed the Discovery channel in via one of the scart leads (from your sky or cable box say) and set your DVD player to channel AV1 or AV2.
Once the DVD recorder is set to AV1 or AV2 (depending which scart socket you use) you should be able to record Discovery.
In fact if you are watching via a Sky box (or cable box) the Sky box or cable box should be connected into the back of the DVD recorder via a scart lead.
Then the scart lead should come out of the DVD recorder and into the TV.
That way you watch Sky via the DVD recorder on AV! or AV2, and can also record while watching it.