East Dulwich is a nationwide stronghold for them, though they are losing suitable nesting habitats due to the sweeping renovation of the old housing stock especially the use of modern roofing methods and materials. If you have had a recent roof conversion it may be worth considering installing a swift nesting box. I think the London Wildlife trust has some free ones. I have some leaflets at my shop, Captured on the Rye with information and contact details for more information. The most amazing fact I learnt about swifts is that as soon as the youngsters are fledged and take their first flight, they don't land again for at least two years, when they first nest themselves. They are truly an aerial species that eat, sleep and breed on the wing. Always great to see and hear them.