I would NOT be surprised if the fasteners are commonly misplaced during an oil change... once the screw is lost, the shop either finds something else that'll thread into the spring nut, replaces the lost screw with the right Jaguar part, or tosses it all for a push rivet or similar DIY level fix (like what happened to the previous owner of my car).

 

Many thanks for the inforamtion on this thread!! the two spring nut fixings that go into my subframe are rotten along with the bolts. Because of the thread I was easily able to track them down here in the uk and order them.
I just spent yesterday fitting a new radiator, waterpump and thermostat on the xk and these fittings were useless. Many thanks again.