And Hugh, great for many other photos where people have tracked down the original spot where old photos were taken or paintings and sketches made. HoL member StephenBln has posted on his Flickr pages some fascinating pairs of photos from Berlin.
Using this technique I can imagine some great comparison shots from across Haringey as well as Alistair's Harringay photos.
It should also be possible to marry it up with an ingenious local history audio project called [Murmur]. This started in Toronto and has also been set up in other Canadian cities; a few in the U.S. - and for a while in Dublin Docklands and Leith, Edinburgh.