For the benefit of all other Apple user's experiencing this frustrating wifi setup issue, I am replying to Linc Davis's suggestion/ advice to me on 25 & 26/10/12:-
It has now been a month of me trialling the suggestion:
"Open the Network preference pane in System Preferences and make sure the WI-Fi connection is at the top of the list on the left. If it isn't, unlock the preference pane, if necessary, and drag Wi-Fi to the top. Then click Apply and test.
...In Network, under System Preferences, the preference pane is already not locked but won't allow the wifi to drag from the second top to the top...
Select Set Service Order from the action menu (gear icon) at the bottom of the connection list..."
So far, this has worked. Thank you Linc and to all who responded.
Kind regards - Richard, for Marmaduke's Macbook Air