Thanks for all the input and offers guys, below is a couple of close up pictures of the fob showing where the little oval button fits.
Mine is marked as Toyota being an import model but the Lexus one is the same bar the logo of course.
Been walking the driveway again but no luck so far, the thing is spring loaded so could have launched into space.
I had noticed earlier that there was a bit of casing missing next to the button and thought at the time I'd better be careful in case the buton flew out, as usuual I put the keys down on the ground whilsts removing all the old MOT stickers from the winscreen and touched the bunch with my size 9s. That's when the thing took off,ironically whilst looking for the button I found the piece that had originally chipped off and caused all the problems!