This is an Adventure Lab Bonus. There is nothing at this spot. In fact, it would be very dangerous to try to attempt to reach it!
You need to complete the Adventure Lab Lord Nuffield and Cowley.
The Adventure Lab app can be downloaded free from the usual on-line stores and is played via smartphone.
https://adventurelab.page.link/iYLc
https://labs.geocaching.com
The coordinates will be revealed as you complete each of the five Adventure labs. Make a note of the bonus numbers from each stage. Once you have seen the final page, you can go back to the opening page and open the "View Journal" section, scrolling right through each of the five pages in turn for a reminder.
The MG reference will be explained in the AdLab itself.
The cache can be found at;
N51° 43.A B C. W001° 12.D E F
Please be discreet whilst finding and replacing.
Congratulations to Rabbithen for FTF.