I've been with Zen for years, I find them to be excellent, especially on the odd occasion where I call the support line. Their technical people know way more than me, which has not been my experience with other ISPs (I do work in IT). I get the full 76Mbps from my service and don't ever notice any slow downs at busy times.
@Furzul, your line was probably suffering from cross line interference. VDSL (BT FTTC technology) is particularly susceptible to this happening when your physical line is bundled with other customers' lines. Essentially, as more and more customers subscribe to FTTC, the interference gets worse and the speeds drop, which is why BT have been introducing something called vectoring. This uses noise cancellation techniques to get your speeds back up, but needs your cabinet to be upgraded to support it.