Success ! It was the BTID that caused the problem as you suspected My now previous BTID from ~20 years ago is no longer accepted and the BTID has to be an email address. I do remember there was a similar problem with the now ceased BT HomeHub VOIP service years ago.
Stephen in Middlesbrough Tier 2 guided me through the BTID change and has now closed the BTIRXXXX4915 & VOL013-XXXXXXXX0233 cases. I am now fully able to see my Billing & Usage following the unexplained loss from May.
I cannot thank the Mods enough for getting this sorted out so quickly and one call from Stephen this evening fixed the problem which has been ongoing for 3 month.
The BTID as an email address is the Accepted Solution.