Phew.. "Found it".
Forgot about the magnetic section on the plug. I grabbed all my spanners together after I'd finished with them and put them back in my toolbox draw, and the blinking thing had stuck to them when I picked them up.
Didn't even occur to me too look in there, until I was reviewing some of the pictures I'd taken of the job and saw the plug in amongst the spanners on my bike lift.
Thanks for looking and replying folks.
