Thanks all. Curiosity got the better of me, and as I was doing some gardening anyway I had a (very careful) dig. The duct goes a good foot down and curves gently away from the house wall - no sign of an end or a junction but I didn’t follow it very far. Appreciate this may be of absolutely no relevance to the install in real life, and I’ll only find out when I get to place an order.

So for now it’s back to the Openreach tracker and watching FTTP availability creep closer!

At a simple level, the method used would be a ‘cut and draw’, attaching the fibre to the existing copper, then pulling the copper cable out of the tubing, thus drawing the fibre along through behind it.
BUT, if there is any kind of snag, then you have now borked the copper and the fibre. With the girth of the copper being so close to the internal diameter of the tubing, making a robust coupling is tricky. Any little kink in the tube will grip the copper and halt the whole process.

Your address is marked as DIG. The guys doing the work will be allocated X amount of time to do Y amount of digging. The viability of the duct100 is something I doubt anyone will be spending much time investigating, especially is it is soft digging.