No, it wasn't caused by wireless interference, I got a sparky friend to have a look before I took it back. It was the power supplies. If you put your hand around the DC power cable, the SNR went up 2db, take your hand away and it went down again. I tried fiddling for hours, putting tin foil around the cables, moving their positions etc. He said it was caused by dirty power supplies. The power blocks that plug into the 13amp sockets were very cheaply made.
I had the same problem with a scanner I once had. When it was plugged in, it caused so much interference that the adsl kept knocking down.
Although the iDECT phone was a mid priced phone, it didn't feel or look as though it was a quality product. Maybe I wouldn't have the same problems if I bought a more upmarket phone. iDECT are made by Binatone. Maybe a Panasonic or other well known make would be better made.
Most people probably wouldn't have any issues, but I know my line is about as long as you get for adsl and the smallest changes seems to make a huge difference.