I don't often go to London but this looks an interesting and action-packed afternoon:
http://www.organistsonline.org/grand.orgue/one.html

Thanks Stephen for bringing to our attention this event - I will come along - looks interesting / entertaining!

David Williams

a bit more expensive is the IAO's London Organ day entitled 'JS Bach and the Lutheran Chorale' in Chelsea early March. I am treating myself.
Details at http://www.pyjhosting.com/lod/flyer.pdf or simply google London Organ Day 2012