Thanks for all the input. Just to confirm, it is definitely a B10A, but it is from ebay, so perhaps it was sold on due to it's average performance, as I'm about to
There is also a variation in sync speed of up to 2Mbps if I reconnect at certain times of day, I assume crosstalk, but could be another factor. I've tried to average the speeds out over several periods of use, but I think my results accurately reflect the general trend. I have had other modems too, an older sagem and a technicolor, both broadcom, and both outperformed by the home hub 5a.
The default snr on my line is 6db as you suspected. For some reason Lantiq is conistantly slightly better on my line, and being able to tweak snr and remain stable is an unbeatable perk. I may pick up anther Zyxel at some point for comparison purposes, but I' quite happy with the vigor as it stands (despite the poor wifi performance, which I remedy with an access point).
Cheers.