I'm with Zen now. Haven't had a single connection issue since the second the service went live.
That's great and Zen is undoubtedly a better ISP. However unless you've moved from ADSL to FTTC (or vice versa) your physical connection won't have changed so the number of connection issues cannot either.
Zen broadband do offer alternative backhaul providers but all FTTC connections rely on openreach hardware in the cabinet and switching providers does not require any changes in the cabinet. It's just a routing table and/or RADIUS update. In fact your modem will likely remain synchronised throughout the switching process.
If you're still on ADSL it could be a switch to an LLUO's hardware though.