I hate to say it but my brief experience with Zen some years ago was just as terrible.
I was getting sub-dialup speeds on my then 8Mbit ADSL.
They were utterly ambivalent to it - they blamed my kit, they blamed openreach, they did everything in their power to avoid trying to fix it. They wouldn't acknowledge that they were replacing a service that ran at around 4.5 - 5Mbits perfectly well for years, just playing the shrugged shoulders game.
Ultimately I exited within the cooling off period. I can't remember who I went to but it was back to normal speeds immediately.
I was genuinely surprised given their outstanding reputation for customer service back then.