As Ethel states, your DVDR takes its signal from the tuner circuit inside your TV. If you turn the TV off, there's no signal for the DVDR to record.
The cheapest solution would be to buy a 'cheapo' Freeview set-top box (for about �25) to feed a signal to the DVDR. This will not only enable you to record programmes with the TV turned off, it will also enable you to watch one Freeview channel while recording another one.
Connect them as follows:
Aerial lead to Freeview box.
Scart lead from the Freeview box to the DVDR Scart input. (***See below)
Aerial fly lead from the Freeview box to the TV
Scart lead from the DVDR Scart output to the TV Scart input (***See below)
***NB: Those instructions assume that your DVDR has two Scart sockets. However, the Amazon description of it only refers to 'Scart socket' in the singular. If there's only one Scart socket, you'll either have to keep switching the plugs around or, easier, buy a Scart switch box.
Chris