Edit: ^ that is the sort of Email address I was thinking of in my previous post. My opinion is: Do that.
What I wrote, pre-edit:
If you're comfortable with the financial side of things, I suppose you could take both routes - order a new 80/20 service right now, AND don't give up on the existing problem. I wouldn't do it, but if you've got the money and patience, then maybe it couldn't do any harm? Also, the non-ceasing of your TT line would ensure that the "Frankenstein line" wouldn't physically be touched (i.e., used as your new line). So then once you have a new, working FTTC service you can decide exactly what to do with the horrible existing mess.
I want a photo of the 6 lines into your house, from the outside, if you go for that, though 

Was thinking about that last night myself. Ill just tell BTOR Im setting up my own call centre at home!
Your thoughts above have crossed my mind, and Flipdee pmed me a way to do it that wouldn't cost a lot.
I think I might go ahead and order a brand new line from a new provider and keep the talk talk connection as it is for now.
If all goes well, this will connect correctly to cab 19 and I will see an FTTC facility available to me to allow me to then order a fit for purpose fibre install. THEN Talktalk can be cancelled.
Does anyone think this new connection wouldn't work, or is the consensus that it is likely too? I just have a fear that the new line works, but this cab 4 muddle still affects things.