I have a baby mattress monitor I don't need any more if anybody wants to buy it off me.
It is two plates that go under the cot mattress and detect if LO doesn't move for 20 seconds. It is so sensitive that it picks up if they don't breathe for 20 seconds and sounds an alarm that is so loud you can't fail to hear it even if you are a sleep in another room.
It runs on 4 AA batteries and one set lasted me the whole time Wil used it (every night for about 10 months)
We bought it because Wil was in his own room from day one and I wanted the extra reassurance. It certainly stopped me checking on him every 10 minutes because I knew if anything happened the alarm would sound.
They are about £80 new I think but I only want £30 plus postage for mine. I would guess at about £5 for postage but would need to confirm.
pm me if you are interested because I always forget to look in here.