My setup is:

2920n router, connected to Virgin Media cable line on WAN2 (working perfectly), and connected to BT Business Hub 3 on WAN1 (not working).

I've set the Business Hub 3 to Bridge Mode, but am looking for some guidance on getting the 2920n configured correctly to talk to the business hub - has anyone else got this configured correctly?

i had my TT normal modem (residential user)at wan 1 and VM to wan 2
The wan 1 you need to set it via pppoe tab under wan>internet access >details page (if is bridged the home hub) and add user name and password.
DON'T forget to click enable at top left of that pppoe page
also maybe the parameters (not remember)
Also you can try if you have dynamic ip click the same ...is if static add it manually
Also don't forget to enable wan > general set up wan 1 >turn it on
Not works like that?

I've tried configuring WAN1 with the PPPOE connection settings, and as dynamic IP as well, but it doesn't appear to pick up at all.

With the PPPOE settings set, I consistently get the "PPPoE : No response from server side" error.

did you add manually the correct vpi/vci/Encapsulating Type,protocol,enable top left page
then left page add username / pass,PPP Authentication add the correct one,fixed ip just tick it (if you have fixed ip)
then down page click ok to save
Had mine perfect working like that when had my 2920n ON

May need to click pppoe pass through option for lan

If you done all that then reboot everything pc,SH,HH,2920 and see if is connecting

sorry forgot is been log time use that settings and the 2920 (now i use the 2850)
the 2920 may not accept all parameters at pppoe mode so check at your HH 3 as well (if is acting as modem) OR the provided modem adsl or vdsl
BT forums are well helpfull as well but i am sure the 2920 can do it
This guide may help you to connect it with other way apart pppoe
http://www.increasebroadbandspeed.co.uk/draytek-2920

Have grown bored of trying the PPPoE connection, so have locked down the HH3, and configured WAN1 with a static IP on the HH3's subnet. Connection made, traffic already going over the BT connection. Job done.

It just feels like cheating...