My Billion router connects to ADSL with no issues and is given an IPV4 and IPV6 address. After about two minutes the IPV6 address is lost and only IPV4 is available. Can you confirm that IPV6 is available on all ADSL lines with BT?
Well, yes, otherwise you wouldn't have got an IPv6 address in the first place. Interestingly, I had the same phenomenon with a TP-Link TD-W9980 that was never resolved, fine with a hub though. Might be worth asking on the Billion forum if others are getting the same.