If error rates improve than after a couple of weeks the DLM will give back speed, my FTTC has dropped to 15 Mbps and then just left it alone and rose again to 19 to 20 Mbps, and one of the lines has risen slightly more to 21 Mbps now on a good day.
The router reconnected again this morning, this time at 66.97Mbps down. (It's still not rebooted though, it just seems to like reconnecting every 3 days).
I will see if I can try moving things around this weekend and see if that helps things stabilise a bit more.