It clearly is available, because you've enabled it before. See the preceding post:
"in the end they agreed to "override the system" and renew my contract for 18 months with 18 months speed boost..."
So, it's just a matter of finding the right person within TalkTalk with sufficient access to the system to enable it.
Please can you escalate it as appropriate.
Good luck with this, Andrew, but in my experience, you'll not get anywhere. You'll get some more senior person phoning you up at home to explain what you've already been told, that the speed boost is not available, even though you had it before and it worked just fine and gave you a faster speed. In my case, initially, I refused to let them close the case because they had not given me what I wanted. But not closing the case means someone will phone you every week to tell you the same thing. In the end, I got fed up with these phone calls and allowed them to close the case but to add a note that I was not satisfied with the outcome. I wish you luck.