installed tracepath6 on asuswrt-merlin, is this indicating its working on ipv6?
admin@RT-AC66U:/tmp/mnt/OPTWARE/entware/etc# tracepath6 www.ripe.net -n
1?: [LOCALHOST]                        0.085ms pmtu 1500
1:  2a02:16c8:0:1::16                                    18.419ms
1:  2a02:16c8:0:1::16                                    16.621ms
2:  2a02:16c8:1:2::19                                    14.764ms asymm  5
3:  2a02:16c8::d                                        14.921ms
4:  2a02:16c8:1:2::8                                    14.988ms
5:  2001:7f8:17::1b1b:1                                  15.336ms
6:  2001:470:0:2cd::1                                    24.364ms asymm  5
7:  2001:470:0:2d0::2                                    22.357ms asymm  6
8:  2001:7f8:1::a501:2859:2                              24.092ms
9:  2001:7f8:1::a500:3333:1                              24.107ms asymm  7
10:  2001:7f8:1::a500:3333:1                              24.134ms !A
    Resume: pmtu 1500

How do you mean?

I mean the pmtu 1500 output from the command is that indicating the ipv6 packets are using a 1500byte mtu?

also I believe we can set a ipv6 specific mtu on asuswrt like this.
edit radvd.conf
Add this line inside the {}
AdvLinkMTU 1492;
Then restart the appropriate service.which I am guessing is done via 'service restart_radvd'
This wouldnt be permanant tho as these files get regenerated on reboots and quite possibly even when restarting ppp in the GUI.

Thanks - that's where the 1480 MTU is coming from
I don't think we can edit the files though as they are written to the nvram - we need to use the source code, edit the file then compile it.
I think the tracepath shows that the 1500 MTU works fine for v6. If it didn't, there would be a lower MTU in the path.

Well the line isnt there on my router so I assume its sharing the main mtu value set on the interface which is 1500.  But that to me looks a way to overide the value in ipv6.

Ok I have an update.
Plusnet booted me today which is I assume to update my profile for me, when I recconected the 3mbit throughput was back, odd I thought, gut feeling made me think its a gateway issue.
Manually jumped gateways and full throughput is back. (note tbb had a bug before where the httpx6 tests on ipv6 were actually ipv4 which I reported which is why before I had full speed on httpx6, now that bug is fixed)
So
On lo0-central10.ptw-ag04.plus.net

and on lo0-central10.pcl-ag03.plus.net