Another vote for Zen. As others say, if you want a reliable bomb proof connection, they supply it. As far as I know my connection has been up and running continuously for 3-4 years. I've never had any access problems. Once I've configured things it just works. When I moved earlier this year, they arranged to transfer the connection to my new property with no issues at all. I unplugged the router, drove down the road, and plugged it in, and it worked straight away, effectively I could run the router at both locations for a short while.
As a plus point, their phones are answer in Rochdale, not Bangalore or wherever it is that I have to ring up when my fathers Tiscali connection has issues. I've nothing against India per se, but there are no language issues with Rochdale, and they have pretty much direct access and knowledge of Zen's systems, they are not simply following a set of questions and answers on a screen in front of them. The one time I had to ring up Zen, when I moved, I also swapped over to paying via Direct Debit, and it all worked easily, flawlessly, and fast.
Definitely recommended, and that's why they've won multiple awards for service over the last few years.
(PCPro Best ISP for 2008, National Customer Service Awards finalist 2008, Which Best broadband provider 2008 etc).