Update for anyone who's reading this.
Murphx responded back to the ISP claiming it was probably my equipment.
Tried with a different modem (DG834GT) in the test socket with PPPoA turned off my BE DSL line to ensure no crosstalk
Wired a laptop directly into the Netgear and disabled the wireless, laptop running on mains power and the only device connected to the router
The laptop was a different machine to the one I tested with originally (Core i5, 6gb ram 120GB SSD win7 x64, Antivirus and firewall disabled).
Also tried with a segmented download manager just in case, doesn't make any difference
So that's 3 different computers, 3 Different DSL modems
Tested with both PPPoE and PPPoA doesn't make any noticeable difference
Still doesn't get more than 16Mbit/s throughput despite a 20Mbit/s sync, sustained throughput is ~14.2Mbit/s
As far as I can tell this is not a DSL fault but rather it feels like a rate-limit being applied somewhere, probably on the RAS / BNG.
Edited by dragon2611 (Sat 13-Aug-11 16:10:39)