I have learned only recently that ADSL24 put us on llu with TTB I think and the speeds were good but before that I was getting a steady 8mbps into the evening also which was far better than what Zen are offering.
Sadly Zen are limited by their wholesale provider, and at your exchange the only provider Zen can use is BT. In BT's eyes they still haven't upgraded your exchange to their new network (21CN / WBC) but still using the old 20CN . Zen's hands will be tied, as BT are suffering congestion on their new network across the whole of the UK, and so I wouldn't be surprised if this is showing up even more problematic on older 20CN exchanges.
Sadly Zen don't offer service using another wholesale network, such as TTB, so you need to find a new ISP who can use a non-BT wholesale network on your exchange.
James BT Infinity 2
19/09/2012 - Sold 42/6 - Getting 49/8.5 - Sync 53 / 9.5 Mbps @ 470m approx
14 years of broadband (ntl: cable to BT FTTC) - Router: Asus RT-N66U - Modem: Huawei HG612 speedtest