When the pump went on my diesel (going about 70mph on the motorway) it took seconds for the temp to rise until the temp light came on, and the throttle pedal was basically switched off.

I pulled over STRAIGHT away and switched it off (infact I switched engine off once i got into the nearside lane, and had slowed to about 35 mph heading for the layby with brakes slammed hard on!! - lucky there wasn't any cars in the way!).

It had spat about half a litre of water out of the expansion tank too.
I got the car home, and checked the water pump.. The impeller had broken off the shaft!

Without a pump, the engine would run on idle for about a minuet or so until the temperature started to rise to 1/3 way up. I didn’t want to leave it running any longer until I fitted a replacement pump, as the temp gauge is slow to respond in the Omega, and will possibly be hoter than it actually says.
Luckily for me, there doesn't seem to be any damage to the engine, and I've since done about 150 odd miles (engine has been running over 12 hrs now, and its not used any water

) - From what I've read, normally damage occurs.