Coming from the West, from the main path find another that takes you towards the river. From the East you could take either the high or low road..or the middle if you can find it. Be sure of Firm Foundations and hunt far...
Watch out for stinging nettles and brambles! If it's dusk or dark you'll need a torch but will be hard to find - better in the daytime. Lots of dog walkers come through here, so watch out for muggles!
Crane Park is a "lightly" managed park with the added attraction of a nature reserve and all kinds of interesting remnants of the old gunpowder mills that used to occupy this area. If you've already looked at the surrounding roads, you'll notice "Powdermill Lane", which takes its name from the old Hounslow Gunpowder Mills.
Once you've found it, take a walk Eastwards along the path. There are lots of other caches in the area so it makes a good trail of discovery - some are quite tricky, but fun and rewarding to discover.
There's lots of history about the area here: http://www.twickenham-museum.org.uk/detail.php?aid=188&cid=2&ctid=2
And a PDF here: http://www.richmond.gov.uk/local_history_river_crane.pdf
Charles Dickens visited: http://www.hounslow.info/libraries/local-history-archives/charles-dickens/charles-dickens-and-hounslow-3/
And good old wikipedia has this to say: https://en.wikipedia.org/wiki/Crane_Park
A word about the "Shot Tower" (not at this cache site, but there is another one close to it). There's some controversy about whether it is actually a shot tower - a place that makes lead shot by pouring molten lead through a grid at some height. There's actually no concrete evidence to support this - the tower isn't actually tall enough for the molten lead to form into nicely spherical blobs before dropping into a pool of water at the base to cool. Research by locak historians suggest it was actually a windmill, used to raise water from the river to the resevoir that used to be at a higher level (seen on old plans of the area). The resevoir was a kind of battery - the water then ready to power water wheels that drove the machinery in the the workshops making the gunpowder.
Hope you enjoy this cache, the park and the many other caches in the area.