Saturday, driving up to some traffic lights, doing about 25 mph. Brakes on and slowing down, just before the car stopped there was a clunk from the back of the car. I was loud enough and jolted the car enough that I thought that I had been hit. My son was beside me, he even asked if we had been hit.

We hadn't and the car has been fine since.

Auto Transmission? Rear Diff? Useless Driver?

Has anyone got any ideas?

Cheers, Hi Darling, I'll be home in 10 mins. If I'm not back, please read this message again...

Maybe you won't remember, but if it was happening exactly in the moment you were switched your foot from brake to throttle, it could be a shudder in the transmission.
It happened to me too, in about two or three situations before, and I believe that when the transmission was switching back to first gear, in preparation for a future acceleration, by pressing the throttle exactly in the same moment, the transmission speeds-up the process, resulting in a harsh change.
I've tried to replicate the process, but I couldn't. It seems that a few conditions should be met to do just that. It seems that it is dependant on the road speed, throttle pushing speed, the moment it changes gears, etc. In both cases it was a slow moving traffic, a lot of start/stops.
So, in my oppinion, don't worry, it's OK, you've just meet/found one of the little bugs of the transmission's firmware. Don't try to replicate it, it wont work... Murphy's law.

Cheers Alex,

from you that is as good as take this pill and call me in the morning!! I guessed that it was the transmission "catching up" Just a little disconcerting!! Hi Darling, I'll be home in 10 mins. If I'm not back, please read this message again...