M25 Jn13: head south as if rejoining M25 and keep in left lane (A30).  Spiral down to Runnymede roundabout and take the third exit (slightly right) to the A308 Windsor Road.  Car park is 1.4 miles on the right where there are islands in the road.  Free to NT members and probably sufferers of ADHD.  Otherwise £3 for 2 hours.

The pub has a large garden and is dog & fox fiendly.  It is just over a mile away.