[IGH]

Thanks to you all for taking the time out to reply.
It may well be a nut from the starter motor.
The nut itself is very clean and looks not to have been under the engine at all (no corrosion or discolouration).
I did have a new starter motor fitted back in October last year.
It is possible they took the nut off the new motor and used the 'old' nut when they fitted the starter onto the car.
Doesn't explain where its been all this time though.
Could it have possibly have ended up on the engine tray and been recovered a few weeks ago when I had some more work done, with it being left in the trough of the N/S wing by the mechanic?
She's been to Bristol twice in the interim, starts on the button and the cruise control was used on the motorway for both trips.
When she next goes in to the garage I will just get them to have a look to reassure me.
Its a bit of a mystery!!!!