I just got an email - they're talking about 02/02/2015 as the installation date, although they'll do it sooner if they can, or later if they have to. The really good news, is that they don't seem to be asking me to pay for the install. I was concerned that I might have to cough up some dosh, because they have to run the fiber from quite a distance away, then from the street into the building. But we found the existing conduit into the building, and they can almost certainly use that for the last part, and I guess they've solved the problem of getting it down the street - there is a conduit for it, but it's pretty old, and the BT engineer was worried that it could be blocked.
So - good news!