Can anyone help with what the WAN settings should be on the TP router?
If I've correctly extrapolated what you've stated (I can't see your setup), you're using the Netgear as a PPPoA router and it's authenticating successfully with your (un-named) ISP. Easy way to check if it's connected OK is to connect a PC on its LAN port (default IP address 192.168.0.1/24).
If the above is correct and functioning as expected, you'll need to set the TP-MR3420's WAN port to DHCP or a VALID IP address in the range 192.168.0.2-254/24, GW 192.168.0.1, DNS 192.168.0.1 or your ISP's DNS servers.
As previously suggested by RockH, leave the TP-MR3420's LAN IP address as 192.168.12.x/24 (anything other than 192.168.0.x/24 - you can't 'Layer 3' route within the same subnet).
CAVEAT: You'll be double NAT'ed.
If you don't want to be double NAT'ed, you'll need to set the Netgear to 'bridge' mode (i.e. it won't be doing the PPPoA negotiation and it won't receive a WAN/PUBLIC IP address) and you should set the TP-MR3420's WAN port to PPPoE (if it's capable) and let it handle the PPPoE negotiation. It'll receive the PUBLIC IP address from your ISP on its WAN port.
**** IMPORTANT ****
If you don't want to be a DLM victim, disconnect the Netgear from the telephone line before trying the following - the Netgear will be rebooted.
And don't reconnect it to the telephone line until you're happy with your setup.
How do you get the Netgear DM111 V2 into 'bridge' mode?
I've no idea, but here's how you do it on a DM111 V1;
login to the Netgear
click setup wizard
select 'No. I want to configure the Modem by myself'
Leave VPI/VCI at 0/38 (I'm assuming that you are using UK specific firmware)
change 'Protocol' to 'RFC2684 Bridging'
then click 'Restart Now'
Sit back and watch the progress screen.
You'll lose contact with the Netgear because the DHCP server is now disabled and your PC is set as a DHCP client, but if you give your PC a VALID static IP address in the range 192.168.0.2-254/24, you'll be able to hit the Netgear's web interface @ 192.168.0.1.
Whilst you're logged in, you could (but I don't recommend it just yet) reconnect the telephone line and watch the Netgear sync with the DSLAM and see if you've been DLM'd.
Theoretically, if you now set your PC as a PPPoE client and enter the correct details, you should be able to connect to your ISP.
Now you need to set the TP-MR3420 WAN interface to connect to your ISP using PPPoE and connect the WAN port to the Netgear LAN port.