"Hic tuus o Tamesine Pater septemgeminus fons" ("Here, O Father Thames, is your sevenfold spring")
The Ordinance Survey and Environment Agency would have that the source of the Thames is located at the aptly named 'Thames Head' near Coates to the south of Cirencester, but this is only one of several possible sources, and it depends on your methodology as to which you will end up at:
Traditionally explorers and cartographers follow the largest stream every time a set of tributaries meet to discern the source, and often this also naturally finds the longest length, but there are some rivers that do not fit this 'rule of thumb'; The Thames is one of them.
If you take an alternative methodology, of following all of the possible streams to find the longest one, you will end up here (or possibly at Ullenwood depending on the time of year) making this the true source of the River Thames (the source at Ullenwood is not as reliable and sometimes dries up, much like the official ultimate source at 'Thames Head'). Seven Springs is, therefore, the longest reliable source of the Thames.
If you take this latter methodology, you will also find that the Thames is 14 miles longer than its official length, making it the longest river in the British Isles, pipping the river Severn by some 9 miles!
Whichever view you take, one thing is for certain: If you were to wee in the waters below you this wee would eventually wash past the Houses of Parliament and under Tower Bridge before making its way to the English Channel.
This cache has been placed to celebrate (but not condone) this fact, and draw attention to the 7 springs of 'Seven Springs' which are directly below it.
Whilst this can be done as a cache and dash, I would recommend taking the (very) short walk down the uneven steps (careful!) to witness the unoffical source of the river thames in all its glory!
Enjoy.