Maybe, the Sky DLM is very different to what other providers use, and not uncommon to see sync rate caps

Cheers! I'll give it a go and see what happens if I lower the SNM. If I can get it down quite a bit and no change, I'll give Sky a ring

In reply to a post by MrSaffron:

Maybe, the Sky DLM is very different to what other providers use, and not uncommon to see sync rate caps

Not just not uncommon, that's exactly what they do.

Your Connection Details
We constantly monitor your line to make sure it has been set to support the fastest speed possible whilst maintaining a stable connection.

Line connection testing completed on 27/08/2012, resulting in the following settings:

Download speed setting
Your connection has been set to a download speed of up to 16.4Mbps .

Upload speed setting
Your connection has been set to an upload speed of up to 1.3Mbps .

These line settings make sure that your line maintains a stable connection.

ADSL Link Downstream Upstream
Connection Speed 16382 kbps 1101 kbps
Line Attenuation 17.0 dB 7.8 dB
Noise Margin 6.5 dB 8.3 dB

They appear to lower transmit power to avoid excess SNR margin - on a fixed 18Mb with a target SNR margin of 7dB I achieve just that while on the capped 16.4Mb I see around 6dB margin at the same times of day.