Not a business customer but work in IT for a living.
As you can see from my signature I've used quite a few ISPs and Zen are by far the best, great service and great support when required.
Virgin (ADSL) => Namesco => Newnet => O2 => Plusnet => Zen => Newnet => Zen Lite 8000
Note: I don't lay turf for anyone. astro or otherwise, all views and opinions expressed are my own based on experience.