Working great.
Line One
BT - Sync 62819/16995 HG612
G.INP Enabled
Line Two
Plusnet - Sync 66074/20000 ECi
G.INP Disabled
Hopefully when G.INP starts on the 2nd line it should jump to 70mbps, i'm currently hitting a cap of 10MB/s I think it's due to the WAN/LAN connection but i'll experiment and see if I can get the TL-ER5120 to manage the whole 120mbps.
All in all very pleased at the moment.
For comparison I've attached the both lines stats on the ECI with OpenWRT.
Edit:
Figured out the 100mbps problem, it was as I suspected initially, the 100mbps LAN on the ECI Modem, as a result of fixing this I am currently downloading a game from steam at 14.8MB/s(118.4mbps).