Seems to be an Apple Message software feature or glitch!
Instead of setting up the handset again as a new phone, which would no doubt clear up the matter, I found this workaround to the problem.
I made a new contact called Xxxxxxxxxx ( I don't have any contacts starting with one X, never mind a series of them! ) and I assigned the old number to that. ( You could even make a contact with the name *!!4££! )
Then I sent a text to that old number.
Then I deleted the new Xxxxxxxxxx contact, and the text message I sent.
The old number no longer troubles me, even though still in the message cache somewhere, because it has been reassigned to a deleted contact beginning with Xxxxxxxxxx, and that's a name I'll never be texting to again.
Great to be rid of what became a real pain, with seemingly no solution, other than setting the phone up again as a new handset.
Hopefully the folks at Apple tech. can sort this in iOS 7!
Communicator