Sorry I didn't come back to this thread before, I was waiting for the site to send me an update email! Oh well.
Anyway, I have one of those machines that make a noise when it picks up a signal, and the noise it made when I held it near our DECT phone was very different and much louder everything else I held it near, even TV sets! This was only matched in strength (but not rhythm) by something the house next door has, which I think might be their baby monitor. But anyway that's only loud in our corridor and has weakened a lot by the time we go into a room. However as we want handsets in many of the rooms, the DECT signal in our house would be in all of them.
I want to buy a new set of DECTs and I would have been interested to try out the ones I read about that were unfortunately not named by brand.