I just received an email from a pal telling me that he walked past the (closed) Mansion House, near Kennington Tube, yesterday, and saw hoardings proclaiming its imminent conversion to an Oakham Ales outlet!
Google found this blog comment, which suggests Old Red Lion may be also worth a look:

http://boggleabout.blogspot.co.uk/20...y-doakaly.html

Great news for an area mostly devoid of worthwhile real ale.