Just to say that over the last two weeks the echo problem hasn't occured once. People I call have not heard their own voice echo back to them. Don't know how or why this has stopped (changed nothing on my PC), but I'm now really pleased with the quality.
Sometimes the call quality isn't so good still - but usually that is because my son is also using the broadband connection to play a networked game. Fair enough.
A couple of days ago I disabled each codec in turn (I'm using the X-Lite software provided) and enabled only the G711u - as has been suggested in other threads. This has also helped voice clarity - at both ends of the call. It sounds more natural, less broken.
Am now finding PlusTalk very usable and a real alternative to traditional phone lines.