The original planned route was a brisk walk through Sydenham Hill Woods from the Crescent Wood Road entrance to the Dulwich Golf Course--before the sunlight ran out--to photograph the amazing country views seen across 2 allotments at the edges of the woods with all the trees in the background.
Halfway towards our first checkpoint (Crescent Wood Road entrance), via Crystal Palace Parade and Sydenham Hill, I remembered to briefly mention to Julia that I had groundbreakingly discovered--after hours of thought and surveillance of the area in the weekends gone by--a way into the mysterious ravine at the side of Mountacre close (see: Sydenham Walkabout (01-Oct-06)).
Given the risk involved, it never crossed my mind for one second that Juwlz would actually dare to take the course in order to reach it, let alone the treacherous course within... "This girl has got some bottle!" said my friend Tony, as we approached and immediately descended the steeply banked ravine before us. 
Our plans for this walkabout had suddenly changed.I realistically doubt we would have made the Dulwich Golf Course in time anyway. With the clocks having gone back the previous night and very little daylight remaining, the photographs I took of the ravine with my Sony 5MP digital camera were too dark to make out any details, but Julia's superior 6MP Canon camera was able to cope much better in these poorly lit conditions to deliver some stunning images.
Julia took the lead and left us blokes way behind!
It was just too damn dark and slippery for Tony and I to properly get inside. Using her initiative, Julia was able to aid her descent with some string tied to a tree trunk, and head off in the direction of Vigilant Close (along the red line shown highlighted on the map below). We could not see her or hardly hear her voice, but promptly shouted to remind Julia to take as many photos as possible. 
Judging from the back of the old people's home (St. Clements) and through a peep hole into the garden of a Mountacre Close house, I have predicted and envisaged the ravine eventually coming to a sudden steep drop; one could imagine this once being a waterfall.
Given the poor conditions, even Julia, whom was complaining about possibly treading inside potential invisible holes through the thick undergrowth, had to admit defeat and return to summit. We will have to go back another time, much earlier in the day, and with better footwear, to fully explore this trail and see if my prediction was true--my prediction about WWIII will have to be false. 
Logical observation of the ravine most definitely points to a stream having ran through it at some point in the past. However, sorry to disappoint you guys, but evidently it doesn't actually appear to be either of the two streams that ran through the Wells Park area. The ravine may not have even been a stream at all--who knows!? But it's certainly a curiosity.
1865/1871 OS map.
1849 Drainage map.
Ariel photo showing the lines of the former two streams by superimposing the two maps above--they fitted almost perfectly--the courses must be fairly accurate.
Download original layered PSD file... (you can see where Beaulieu Avenue was built in relation to Longton Grove and the old Westwood Hill houses--for example--by shifting the opacity/transparency of the two maps)
The light blue lines are from the 1865/1871 OS map and the dark blue lines are from the 1849 Drainage map.
The upper stream started right by the ravine, but seemingly didn't take the course through it, and instead went in the north-eastern direction of Mountacre Close before coming downhill through some woodland across the end of what is now Vigilant Close where Pat mentioned a house had subsidence. It then went across Longton Avenue and Wells Park at points we've previously predicted, and finally across where they dug the pond +/- a few metres. The coverage stops here, but you can see the stream definitely missed the first allotment, but the second allotments are in perfect line for it.
We've now also got some idea where the lower stream used to reach--nowhere near the high banks of high level drive/Hillcrest Close where I predicted.
Map showing the ravine we visited today. The circled house is way before vigilant close--more like the corner of Talisman Square.
Julia prepares for descent as seen from my camera; all the rest were taken with hers (click for original size).
Looking back at Sydenham Hill.
Looking forwards towards Vigilant Close.
Looking backwards.
Looking forwards.
Looking forwards with the flash enabled.
Looking forwards without the flash.
Inside the dark ravine at an unknown angle.
Looking forwards with a building on the right-hand side (circled on the map above). Still a long way to travel before reaching the end.