Is it possible they might be willing to use the ducting? But I'd imagine there are certain standards it would need to conform to.
After all, its not the fibre that is the expensive part.
This absolutely. The trouble with using your fibre isn't actually technical but about who's responsible if something goes wrong. They need a demarcation point between the home installation, the bits that are your problem, and theirs. Using your fibre blurs that as your fibre is feeding their equipment.
You put a JCB through the duct and cut their fibre you repair the duct, they repair the fibre and send you a bill. 😊
The only complication is that it isn't just one person the duct is serving. Previous examples of this I've seen the duct was down a private drive. That basically meant all on one person's property so it's like one giant drop. In this case it's a part of their access network serving multiple people. They'd have to 'adopt' it and other than Openreach and VMO2 I'm not sure if any others would be prepared to do that.