I want to record a show on a Plus 1 channel but my recorder won't let me.
Recording from the guide (or EPG) uses something called "alternate instance" which always chooses the original / first screening of the content selected to record - it sees the same programme on the Plus 1 channel as being identical to the one on the original channel and would always opt to record the show on the original channel (unless there is a prior conflict in your booked recordings).
The advantage of recording off the original channel is that broadcast quality is often better, leading to better quality recordings for an enhanced viewing experience. For example, most prime time shows on ONE, TV2 and TV3 are broadcast in HD on Freeview|HD; the same shows on the Plus 1 channels are not.
However, if you do wish to record from the Plus 1 channel, you can do so by manually setting the timer. Please refer to your device manual for more information.