We've got a Byron, I think we got it from Homebase.
It's not like you describe but I think it's better - the main unit is by the front door and plugged into a socket, but there's a second free-standing pocketsized remote unit which is battery operated, which is currently upstairs in our living room, but in the summer we can take it out into the garden, anywhere in range of the base unit. We've had it a couple of years now, I think we've had to change the batteries only about 3 times since then.