Last night I discovered that we have at least 1 rat beneath the floorboards- the gnawing gave it away!
Anyway, I've found a hole in our wall where the telephone wires for the flats above come out and it leads straight into the void under our flat.
The question is, do I block it with wire wool and expanding foam first, before lifting up a couple of floorboards to lay poison ie probably trapping the rats beneath the floor [grim emoticon]? Or do I put traps down for a night and then block it up, which may allow more rats in? 
Also has anyone tried a electronic rat zapper or a plug in repeller before?