What 20 min alarm?

do you mean the alarm if baby does not take a breath in 20 seconds?? This only goes off if baby holds breath, stops breathing or wriggles of pads.

DH also didnt want to co sleep or baby in room but in hospital DS was in bed with us some nights.

We bought a new one as new model was only released 2 weeks before we bought it and also will use for 2 babies so figured was worth money. You could get a 2nd hand but I would want to pay no more than 1/2 price of new otherwise would get a new one.