Thanks for everyone's help on this. Eventually got my FTTP order placed last night.
After trying to Full Fibre Team number all day and getting nowhere, I eventually tweeted BT and a very helpful Laura phoned me back within 2 minutes and forwarded my call to Ryan on the Full Fibre Team.
He could see that I could get full fibre as a new customer but not as an existing one - he said it happens a lot. He didn't know about the SoGEA issue but as I said it to him he did seem to suggest that was the common factor in all the accounts it happens to. So if anyone else is having that issue, it's definitely worth perusing that line of attack.
The solution - they've put my order through as a new order and are cancelling my existing contract once it's in place, waiving the early termination fee. Best of all, because it's going through as a new order, I'm getting it at the new customer price - something I wasn't expecting because I'd been told previously that upgrades would likely not get the benefit of this.
Roll on the end of November - 18meg to 900 meg should make a huge difference!
