Well it was worth a try! - No can do on the 'lifting the floorboards upstairs' - bathroom heated floor with tiles. I think we'll just have to knock out the cement and a bit of the plaster wall above to expose more of the wire and then call the electrician. Thanks anyway.
:oops: We discovered a box on the wall whilst decorating which was once a light switch we assumed. As we no longer needed it (could see where door to outside used to be so thought maybe outside light!) we taped ends of the wires and cemented in the hole to make smooth for wallpapering (did this...