"Christian McArdle" wrote in message et...
Use a stud detector to find the noggin. (These are the short horizontal
members. There will be one or two in the wall). If you haven't got a stud
detector, drill a hole near where you suspect it may be from tapping. Insert
a piece of stiff earth wire bent at a 90 degree angle. Twist the wire and
feel around until it hits the noggin and you can work out where it is.

(etc.)

Thanks Christian, I think I'll go with this.
And thanks to all other respondants, too. I'm grateful for the suggestions.

John