Managed to break into my Fritzbox 7390 and reset it. It seems my fiddling may have put it into Wi-Fi extender mode, which explains it shutting down as a router yesterday. Running nice now on IPv6 and no dropped packets this time on the latest firmware FRITZ!OS 06.53 if that helps.

Thanks, and it's good to know that the Fritzbox 7390 is working correctly on A&A's implementation of IPv6.

What may be a problem is that I believe that BT Consumer have configured IPv6 somewhat differently to A&A. To date I have only read of those using BT's SmartHub as being able to correctly run IPv6 and even then I seem to recall that when I was using the SmartHub it only scored 9/10 due to problems with BT's IPv6 DNS server, or rather the lack of. I'll drop the SmartHub back on over the weekend, make a note of such settings as are visible and at the same time let the firmware update to the recently released SG4B1000223E.