Thanks for the feedback everybody. To answer the various questions, the easy one is that the ISP is Demon.
Running my router with default settings this morning I get
Downstream: 1281kpbs, Attenuation 63.5, Noise Margin 10.5 Upstream 554kbps, attenuation 31.5 Noise Margin 15.0
I tried forcing different modes, with adslctl configure --mod, but the only options that would sync were the a and d modes, and the figures were pretty much the same.
I once again tweak the SNR setting and have now taken it down to 5 and get
Downstream: 2043kbps, Attn. 63.5, noise margin 4.4, Upstream 554kbps, Attn. 31.5, Noise margin 14.0
However it is not very stable with an SNR of 5, so I moved to SNR 7 and I get
Downstream: 1973kbps, 53.5dB, 4.8dB Upstream: 554kbps, 31.5dB, 14.0dB