I have a Vigor 2820 with WAN1 line giving about 1.4Mbps with line attenuation of 51dB and SNR of 10.
My second line for WAN2 via a BT 2-Wire runs at 4.2mbs with line attenuation of 49 dB.

Is my difference likely to be due to Vigor or my line or BT?
Vigor is runing
Model Name : Vigor2820Vn
Firmware Version : 3.3.5.2_232201
Build Date/Time : Apr 7 2011 18:10:43
ADSL Firmware Version : 232201_A Hardware: Annex A

Line 1 is on what kind of service ? "Up to 8 Meg" does it connect at G.DMT or something else ?

Line 2 is with a different provider, or the same? What kind of service ? Perhaps it's an ADSL2+ connecting at AnnexM ?

Sometimes the Drayteks they don't always detect (using Multimode) the fastest available negotiation for your line - if this is the case you can select it manually.

Both lines are BT and connect on G.DMT up to 8meg lines

Try the 211801 modem code, that should improve speeds a little on WAN1