Is it possible for the fibre cable to be routed via the eisting telephone cable ducting under my garden, and use the existing cable entry point?
As long as the Duct is a Duct 56 and isn’t blocked or Collapsed then that would be the preferred method on putting in a new Fibre Cable anyway for Openreach.
As long as the duct isn't blocked, that would be the normal method of delivering the cable.
The price of a given speed of broadband is the same regardless of whether delivered by fibre or copper. Only the higher speeds unable to be achieved by copper are more expensive.
They don’t ( or shouldn’t ) use the copper cable to pull in the fibre cable, there is no reason why both couldn’t exists together in the same duct, it’s possible that if you can change back to a provider that will use the copper pair , even though fibre is available , eventually that option won’t exist.
Duct 56 is a Grey Pipe that 2.5 inches wide.
They’ll ‘Rod and Rope’ the new Fibre Cable through using a Cobra and or Hand Rods.
Although in some circumstances Openreach will use the existing Copper Cable as draw cord.
They will only do it as a last resort though as there’s a chance you’ll end up with nothing.
The copper telephone cable emerges from a grey plastic pipe about 50mm inside diameter. Some gravel has fallen down it but it is possible to move the cable in and out so only a loose partial blockage at the house end; hopefully, this will not be a problem.