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Under the car today and noticed that there was a small leak of fluid. Texture was oily, colour was yellow/orange. The location was the rear corner driver side hex nut, which I think is holding the gearbox cover in situ.
The car is an XKR, I wondered whether this was a gearbox leak or some other fluid.
Any thoughts welcome

It's always difficult to be sure about fluid sources because, quite naturally, they have the ability to move! However, the location does suggest the gearbox may be the source. Have you tried cleaning around the area, running the car and then rechecking the evidence to pinpoint exactly from whence the leak emanates? If it is a gearbox leak (as opposed to a seep) it should be fixed quickly or other symptoms and possible gearbox damage will occur. Mike.

I wonder now if it is the dye put in the cars air con system by the dealer to check for leaks

If they're present in any quantity (enough to spot on the floor) the stain trail of aircon dyes can usually be followed back to the leaking component even without a light to make them fluoresce. Mike.