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?
If it wasn't you wouldn't have got an IPv6 address in the first place. I suspect the problem lies with the Billion, I had exactly the same with a TP Link 9980 with beta firmware. TP link never did produce firmware that worked with BT's implementation of IPv6 for the 9980. If you can borrow a HH5 or later with latest firmware, you can verify if that works or not whether it is the Billion at fault.