Petts wood, Chislehurst and hayes are 3 good places to go. Been out exploring most weekends and found some nice little trails through the woods. plenty of singletrack, some mad little downhill sections, pretty basic but fun anyways (and good enough for bromley)
Basically you need to find thornet wood in petts wood or chislehurst woods (in chislehurst!! lol) on the map......
have a look here http://maps.google.co.uk/maps?q=petts+wood&ll=51.390637,0.074501&spn=0.063194,0.185257&hl=en
if you do the satellite thingy you'll see what i mean.