[Mr Gazza]

Thanks again for flagging up a harmless cheapie... Anything has got to better than no alarm.
It never occurred to me to just point the remote under the tank. That's a nice simple solution to an already nice simple idea... The logic is just stacking up in it's favour. (Can you actually "stack" logic? Or is it the name of a new type of play brick, like Lego, from China?)