Hi,
Hope it's been of some help to you.
I was told that my house didn't have cavities, firstly by the inspection guy at the council, he used google street view to look at the property (!), then the guy who came to size up the house to have the cavity wall insulation done said they looked like the ones with no cavity, I said that I had drilled through it myself putting an outside socket on and know I went through a cavity. He drilled various test holes and stuck his bendy telescope thing into the holes, and of course there it was, the cavity.
So don't let anyone say you don't have cavities without them checking PROPERLY first!