You're looking for a cache hidden within 5m of the co-ordinates given. May not be exact as it was very difficult to get a fix.
Edit: The cache went missing in early March 2012 and was moved to a more protected location about 2m from the previous location. It is still just as challenging and within 5-6m of the published co-ordinates. Good luck!
It is accessible after a short walk from Sheffield Bottom Car Park and does not require venturing out into the middle of the M4!
This cache is NOT suitable for children. Please note that this cache is rated difficulty 5 - please do not expect to find it easily, quickly or without a good deal of experience. Good luck!
*Please, NO spoilers in your logs!*