A no nonsense, no frills micro with just a log book and pencil (no swaps please). Set just off junction 6 (Lewknor) with easy and obvious roadside parking at the foot of the northbound exit slip. A short walk down a few shallow concrete steps, so suitable for the smartest attire. If you need a pit stop, you could do a lot worse than visit Ye Olde Leathern Bottle in the village, opposite the end of the lane just beyond the cache. No fuel here, though.
For a full list of Motorway Mayhem caches check this web page.