Doesn't seem like a bad deal there Ben, I'm currently on F2S, which doesn't seem too bad. previously I used eclipse, which were good, but better deals made me defect - before that, it was pipex, who now seem to be offering a few reasonable deals - the service was good, nearly 2 years with only the 1 minor outage.
As nelz and others say, Zen do have a very good reputation. A bit dearer than most, but I suppose the smaller ISP's can't compete on price so they make up with excellent service/customer support (one of the blokes at my LUG is a debian developer - his own mirror etc etc, rates them very highly).
Maybe you should just compare at [url]http://www.adslguide.org.uk/
[/url] - they always seem pretty on the ball about this.
"What the large print giveth, the small print taketh away". Tom Waits