We need further information, Galasalmon.
What type of recording device are you using? VCR? DVD recorder? PVR? (The make and model might help as well).
What else have you got? A Freeview box? A Sky box?
In the 'good old days' of analogue TV, you could record one programme while watching another because you'd got two TV tuners in use simultaneously. The one in your TV set fed its output to the TV screen, while the one in the VCR fed its output to the tape. Now that we've moved on to digital TV you'll need two digital tuners within your set-up in order to be able to record one programme while watching another. Without the information requested above we won't be able to tell whether you've actually got two tuners and, if so, how to arrange things correctly for recording.
Chris