This was a 10 inch, chrome vandium ratchet extension. It was left sitting on top of the engine!!
I noticed when cleaning the car some 5 days after the service, as I wiped down the engine bay I dragged it out with my cloth. Worrying, especially as 24 hours earlier I had wound the car up to about 110mph. Imagine if that had fallen into the pulleys.
I've still got it; they can have it back when I get my free service.