Not again. In all my years - 4 of them resident at Hammersmith - I never once witnessed one off the road. All of my then colleagues did, and it has certainly been a regular occurance before and since my time there.
Being on leave last week and next I cannot shed any further light on it, but I would imagine one of my intranet reading colleagues will be along shortly. More than likely a low speed derailment, but with the layout there, any small glitch can shut the whole depot down. The Districts sent from Hammersmith will normally be the first cancelled due to the stock being stuck in depot (with any stock able to be got out of depot used for H&C or Circle services first.)