Thanks all - so far! I will try some of these suggestions later on and report back.
BN. I switched off my home-hub at the wall socket and rather obviously lost the wifi signal to my laptop and iMac. I was able to connect to the BT-fon signal by entering my BT username and password and thence surf the internet etc (bbc.co.uk for example).
Norton did warn me that I was connected to an insecure network.
Would tend to suggest the wifi signal is airborne outside my house if my hub is fully powered off, but more than willing to be educated further on this - maybe something powered by the incoming telephone line for example?