Update 20 January 2015.
In discussion with the Technical Support team I was advised that the process of transferring an existing sub account into a primary account does work but there is an issue in making the original primary account into a sub account of the 'new' primary ID.
Work is on-going to resolve the issue.
It does mean that the creation of new sub accounts is not possible at this stage.
Had email from Technical Support team yesterday 26/1/15 advising that work was in progress.
Have checked today 27/1/15 and now find that the requested work is complete - the original sub account is now a Primary account, the original primary account is no a sub-accout of it and another sub account has been transferred as well.
Not so easy to find was the facility to add new sub accounts but I got there eventually from an old link - could not find it anywhere on the current MyBT or BTYahoo Home pages.
Will consider problem solved and thanks to those involved.
In conclusion, a couple of weeks on, have deleted the original "primary" account which was soft (just a few lowercase letters) and receiving lorryloads of spam and reset some of the parameters in mail accounts and all seems OK.
Spammers no doubt getting their rubbish back and so they should.
Bottom line - was hard work and took some time, most of which was finding the persons who knew what to do!
Guys in Executive Complaints section were excellent in that respect - and thanks to them.