Finally went back in yesterday to yet again demonstrate the noise, wen out with two master techs in the car and the damn thing wouldn't make any noise at all after 30 mins driving and me trying to get it to do it. they basically forced me to take the car away and book it back in 'when it gets worse' and we can hear it more apparently.
Car was fine all the way home from the dealership, even had me thinking it was somehow resolved, then 20 minute drive in the evening and it's making the same noise as usual.
I did get a bit more info on the rattle under the car however, it's a TPI for this exact model and they fitted a plastic shielding around the gearbox casing to reduce the noise but not eradicate it, the resulting noise is deemed acceptable - this is what was fitted back in September 2017.
I give up.
Something to bear in mind, is that the 7 speed dry box will rattle at low speeds (I never hear it in the cabin but windows down its audible). My Ibiza Cupra did it in 1st/2nd when pottering around as the gearbox lets one of the clutches lose. My other halfs Leon 1.2tsi DSG does it, but my Leon Cupra DSG doesn't as this uses the 6 speed wet clutches.
Just something I thought of.