The IPV6 connection type normally is decided by the ISP. I think you can try to use IPV6 PPPOE prefix delegation and then see if you can get the IPV6/48. On the TP-LINK page, you can several option, you need confirm with your ISP to see which one you need choose.

Sorry for the delay in replying - I didn't get an email notification :-/

I've tried that and it never gets a connection. In fact I lose my IPv4 connection as well. It looks to me like this option is establishing a new PPP session for IPv6 and failing. In the process it knocks out my IPv4 session. Here's part of the log from my router:

####################################################################
# TL-WDR3600 System Log
# Time = 2013-09-01 9:03:01 483407s
# H-Ver = WDR3600 v1 00000000 : S-Ver = 3.13.31 Build 130320 Rel.55761n
# L = 192.168.0.1 : M = 255.255.255.0
# W1 = PPPoE : W = 0.0.0.0 : M = 0.0.0.0 : G = 0.0.0.0
####################################################################

Sep 1 09:02:10 PPP WARNING LCP down
Sep 1 09:02:10 PPP INFO send_phase 2098 pppd_phase = 0x4
Sep 1 09:02:10 PPP INFO In pppd the httpd-id is 666, set link phase is 0x80000004
Sep 1 09:02:10 PPP INFO sent [LCP Req mru=1480 magic=0x7837cbc5]
Sep 1 09:02:10 PPP INFO sent [LCP Rej mrru=1524]
Sep 1 09:02:10 PPP INFO rcvd [LCP Nak mru=1500]
Sep 1 09:02:10 PPP INFO sent [LCP Req magic=0x7837cbc5]
Sep 1 09:02:10 PPP INFO rcvd [LCP Req auth=chap-MD5 magic=0xf12b439a endpoint]
Sep 1 09:02:10 PPP INFO sent [LCP Ack auth=chap-MD5 magic=0xf12b439a endpoint]
Sep 1 09:02:10 PPP INFO rcvd [LCP Ack magic=0x7837cbc5]
Sep 1 09:02:10 PPP INFO send_phase 2098 pppd_phase = 0x5
Sep 1 09:02:10 PPP INFO In pppd the httpd-id is 666, set link phase is 0x80000005
Sep 1 09:02:10 PPP INFO rcvd [CHAP Challenge (ddbffd67f22559028e7e26a3c3a1c190), name = "IDNetLNS"]
Sep 1 09:02:10 PPP INFO sent [CHAP Response (154e50f0bdff6cadde756e1f5e3c4893), name = "xxxxxx"]
Sep 1 09:02:11 PPP NOTICE call pppoeFormatCmd
Sep 1 09:02:11 PPP NOTICE Standard authentication.
Sep 1 09:02:11 PPP NOTICE g_cmdBuf:pppd pppoe eth0.2 defaultroute unit 0 noaccomp nopcomp httpd-pid 666 user_len 17

passwd_len 8 special-flag 100 mru 1480 mtu 1480 usepeerdns
Sep 1 09:02:11 PPP INFO send_phase 2098 pppd_phase = 0x2
Sep 1 09:02:11 PPP INFO In pppd the httpd-id is 666, set link phase is 0x2
Sep 1 09:02:11 PPP INFO sent [PADI Host-Uniq(0x000002a1)]
Sep 1 09:02:15 PPP INFO sent [PADI Host-Uniq(0x00000287)]
Sep 1 09:02:15 PPP INFO sent [PADI Host-Uniq(0x0000028e)]
Sep 1 09:02:15 PPP INFO rcvd [CHAP Success ""]
Sep 1 09:02:15 PPP INFO send_phase 2098 pppd_phase = 0x7
Sep 1 09:02:15 PPP INFO In pppd the httpd-id is 666, set link phase is 0x80000007
Sep 1 09:02:15 PPP INFO sent [IPCP Req addr=0.0.0.0]
Sep 1 09:02:15 PPP INFO sent [IPV6CP ConfReq local Peer Ipv6 Address = fe80::3cb4:8539:c800:8b97]
Sep 1 09:02:15 PPP INFO rcvd [IPCP Req addr=212.69.63.98]
Sep 1 09:02:15 PPP INFO sent [IPCP Ack addr=212.69.63.98]
Sep 1 09:02:15 PPP INFO rcvd [IPV6CP ConfReq remote Peer Ipv6 Address = fe80::523d:e5ff:feb7:e718

ip=fe80::3cb4:8539:c800:8b97 fe80::523d:e5ff:feb7:e718]
Sep 1 09:02:15 PPP INFO sent [IPV6CP ConfAck ip=fe80::3cb4:8539:c800:8b97 fe80::523d:e5ff:feb7:e718]
Sep 1 09:02:15 PPP INFO rcvd [IPCP Nak addr=xx.xx.xx.xx]
Sep 1 09:02:15 PPP INFO sent [IPCP Req addr=xx.xx.xx.xx]
Sep 1 09:02:15 PPP INFO rcvd [IPV6CP ConfAck ip=fe80::3cb4:8539:c800:8b97 fe80::523d:e5ff:feb7:e718]
Sep 1 09:02:15 PPP INFO send_phase 2098 pppd_phase = 0x8
Sep 1 09:02:15 PPP INFO In pppd the httpd-id is 666, set link phase is 0x80000008
Sep 1 09:02:15 PPP INFO rcvd [IPCP Ack addr=xx.xx.xx.xx]
Sep 1 09:02:15 PPP INFO ipcpup : ipcp_wantoptions[0].default_route = 1^M
Sep 1 09:02:16 PPP INFO sent [PADI Host-Uniq(0x000002a1)]
Sep 1 09:02:20 PPP INFO sent [PADI Host-Uniq(0x00000280)]
Sep 1 09:02:20 PPP INFO sent [PADI Host-Uniq(0x00000279)]
Sep 1 09:02:20 PPP INFO sent [PADI Host-Uniq(0x00000272)]
Sep 1 09:02:20 PPP INFO sent [PADI Host-Uniq(0x0000026b)]
Sep 1 09:02:21 PPP NOTICE call pppoeCmdReqIPv4
Sep 1 09:02:23 PPP NOTICE call pppoeFormatCmd
Sep 1 09:02:23 PPP NOTICE Standard authentication.
Sep 1 09:02:23 PPP NOTICE g_cmdBuf:pppd pppoe eth0.2 defaultroute unit 0 noaccomp nopcomp httpd-pid 666 user_len 17

passwd_len 8 special-flag 100 mru 1480 mtu 1480 usepeerdns
Sep 1 09:02:23 PPP INFO send_phase 2098 pppd_phase = 0x2
Sep 1 09:02:23 PPP INFO In pppd the httpd-id is 666, set link phase is 0x2
Sep 1 09:02:23 PPP INFO sent [PADI Host-Uniq(0x000002dc)]
Sep 1 09:02:25 PPP INFO sent [PADI Host-Uniq(0x00000287)]
Sep 1 09:02:25 PPP INFO sent [PADI Host-Uniq(0x0000028e)]
Sep 1 09:02:26 PPP INFO sent [PADI Host-Uniq(0x000002a1)]
Sep 1 09:02:28 PPP ERROR User request
Sep 1 09:02:28 PPP INFO send_phase 2098 pppd_phase = 0x9
Sep 1 09:02:28 PPP INFO In pppd the httpd-id is 666, set link phase is 0x80000009
Sep 1 09:02:28 PPP ERROR LCP down
Sep 1 09:02:28 PPP INFO send_phase 2098 pppd_phase = 0x7
Sep 1 09:02:28 PPP INFO In pppd the httpd-id is 666, set link phase is 0x80000007
Sep 1 09:02:28 PPP WARNING LCP down
Sep 1 09:02:28 PPP INFO send_phase 2098 pppd_phase = 0x4
Sep 1 09:02:28 PPP INFO In pppd the httpd-id is 666, set link phase is 0x80000004
Sep 1 09:02:28 PPP INFO sent [LCP TermReq]
Sep 1 09:02:28 PPP INFO rcvd [LCP TermAck]
Sep 1 09:02:28 PPP INFO send_phase 2098 pppd_phase = 0xa
Sep 1 09:02:28 PPP INFO In pppd the httpd-id is 666, set link phase is 0x8000000a
Sep 1 09:02:28 PPP INFO send_phase 2098 pppd_phase = 0x0
Sep 1 09:02:28 PPP INFO In pppd the httpd-id is 666, set link phase is 0x80000000
Sep 1 09:02:28 PPP INFO sent [PADI Host-Uniq(0x000002dc)]
Sep 1 09:02:31 PPP NOTICE call pppoeCmdReqIPv6
Sep 1 09:02:31 PPP NOTICE Standard authentication.

My case was passed onto a senior engineer. He's looked at a Wireshark log and says that my ISP uses RADVD and the WDR3600 doesn't support it. I thought that was the case - it doesn't seem able to share a single PPP session between IPv4 and IPv6. He suggests that it might in a future firmware upgrade and I hope for TPLink's sake it does. It seems to be a very common way of implementing dual-stack IPv6/4 - certainly in the UK anyone using the encumbent's FTTC or ADSL service will be using it. At a guess I'd say that the WDR3600 for all their claims of 'IPv6 ready' should not be making that claim in the UK market.

I think it was very telling that when I used my computer to connect to generate the Wireshark log Windows 7 got a dual stack session without batting an eyelid. Just the default 'dial-up' options and it connected immediately. So Windows 7 supports native dual stack with my ISP out of the box without needing any special configuration. That doesn't say much for the WDR3600 in my opinion.