It is shown on maps going back to 1862 but there appears to be differing stories about it. It's possible that Tonge House was on the island and part of the Portal family lived there. HJ Portal was born there in 1845.
Rumours are that this was bombed in WWII and replaced with the flats but one of the trustees of the area apparently thought it was (probably) built in the 1930s with money donated from the then Lady Portal. So the bomb theory may be piffle!
Either way it's pretty impressive and I would imagine that it's a sought after address.
This is an easy park and dash 35m film cache.
UPDATE: This cache has been found about 150 times and very popular but unfortunately I no longer go anywhere near the area having stopped working in Streatham (after over 30 years, on and off) teemarie3 has kindly offered to adopt it. Jon.