Friday night was a HellCommute. Terrible traffic from Sutton to the M25, explained by the closure of the slip road onto the M25. After that? A 90 minute or so diversion around Crawley and Horsham before I finally joined the A23 and made my way home. I never got around to finding out what happened, until I read this on Kat Arney's blog this morning:

Unfortunately Chris and Mary never made it. They’d got stuck in the worst traffic jam on the M25 for years, and ended up motionless for four or five hours.

The worst traffic jam on the M25? Ah-ha:

The M25 in Surrey has reopened in both directions after being shut for more than 24 hours following a crash in which a lorry hit the central reservation and collided with a car.
Three people were badly hurt in the crash, between junctions seven with the M23 and eight with the A217, on Friday.

Mystery solved. The internet: bringing me the things I forgot to look up.