I wonder if any of the Openreach guys on here know if stores item 066165 (duct 56 seal) is still available as it may be the best solution for the OP once the fibre cable has been put through the ducting between the front of the property and the under stairs cupboard. I know normally the default for protecting ducting is a 'Duct Plug 1A' but the duct seal should protect against the ingression of water and gas (and hopefully rodents).

Edited by deleted (Fri 11-Nov-22 22:51:23)