Thanks for all the inputs. I think I'll give the Fritz!Box a try, at least I'll be able to see some basic line stats and it is considerably newer than my OR ECI.
Thanks for the link to grafana, I've not come across that site before, it looks useful. I should probably put some effort into getting some proper line stats since I really have no idea at the moment if there is something weird on my line. I am on the lower end of my line speed estimates according to BT.....Clean is estimated at 32 to 45Mbits/s and impacted is estimated at 22 to 37Mbits/s.
I have got my fingers crossed for a DLM reset, but I guess I can always ask Zen for one if it doesn't happen. I've not really worried about the lower speeds since ASUSgate as long as it's stable. However I've got recently upgraded my TV and could do with a bit of extra speed for 4k streaming.
I have looked at the Draytek and Netgear VDSL modems, but I think these both use Lantiq chipsets. I might give the Draytek a go if I don't get on with the Fritz!Box. Are there any stand-alone VDSL modems that have Broadcom chips?
I've currently got my router setup to use baby jumbo frames (RFC4638), which works fine with the old ECI modem. Does anyone know if the Fritz!Box, Draytek or the Zyxel's support baby jumbo frames? Not really a problem, just a nice to have.