Quick update ;
I've aired on the side of caution and booked into Mann Egerton VW in Swindon for the DSG oil change tomorrow.
However, for the key, I've found an independent Elec Key specialist in Swindon that works mobile, so will come to my work, carries genuine VW parts, can cut, program and set to my car a new three button key for £120 all in.
For my own peace of mind, he'll also 'decouple' the lost key from my car so that it will no longer disable the alarm or immobiliser within the cost
He came very highly recommended by a bunch of people when I asked amongst my friends, and is coming this afternoon, so I'll report back later as to whether I've saved £50-60 and a couple of hours off work getting the car to and from the VW dealer (they couldn't do at the same time as DSG change as apparently they don't hold stock of blank keys and it's 2wks L/T)