£850 and a week later, the car is back ... Having said that, the £850 weren't for the rattle but a new grill and radar mount. Someone (me) may have rear ended someone :-/
Car only came back today as of half a dozen parts needed to fix that - a single bracket couldn't be sourced in the UK so had to come from Germany - but it gave the technician quite some time to test that rattle. And yes, on the way back home, not a single noise ... The whole car seems quieter (in a good way) ... Tomorrow is school run and the way to school has usually a 99% success rate of rattles ... So we shall see.
But yea - if anyone got a rattle that you cannot locate at the front, check air vents ..