So, as suspected install day came around and things didn't go to plan. The install itself actually went ahead, nothing flagged up on the Openreach system beforehand.
The engineer didn't have to do anything in the house, other than swap the Mk4 filter for a gfast one and made sure the modem was powered on and plugged in. He did his bit in the cab, confirmed it was all setup, then waited for the modem to sync. And waited. And waited some more.
In the end he did a pair swap, a port swap but it always resulted in no-sync at the house. After a few hours (!) he called it in and only then was it confirmed it can't be done. Good effort from the engineer, and he was willing to try everything to try and get some life out if it Although I did find out the line length is around 450m, useful to know!
He also commented I was the only one hooked upto that pod at the time - clearly showing what a ludicrous plan it was to install to this cab.