I sealed our ducts up this way:
First poke a ball of expanded metal down as a rodent barrier.
Next shove a plastic bag round the cables/pipes, deep into the duct, with the open end upwards.
Finally, squirt expanding foam deep inside the plastic bag, trim the top flat and add a bit of airtightness tape for good measure.
This sealed things up very well, and also had the advantage that it could be undone. I needed to pull the length of 25mm² SWA further into the building, when I decided to relocate the CU upstairs, and found that it was easy enough to pull the foam-filled plastic bag out of the duct, because the foam hadn't been able to adhere to the cable or duct.