wrote in message
...

So am I right in thinking that (roughly speaking) for continuous running a
conventional system is more efficient,

No. The hybrid is equally as efficient on say motorway runs, if not
better.. In towns conventional then fall apart in efficiency.

whereas doing a lot of shuffling
along in heavy traffic a hybrid can be better?

A hybrid is infinitely better as it claws back kinetic energy when braking.
They are also brilliant to drive, unlike manual tractor like diesels.

Has anyone done the figures
(for current production technology conventional and hybrids) to work out
which would be better averaged out over national traffic patterns? In
other
words, if you were to replace the entire national car fleet overnight with
equivalent hybrids would net fuel consumption increase or decrease?

Well Prius and Honda users are getting an average of 60mpg, with vastly
reduced emissions in towns. The Prius is largish car, with an interior the
size of a Camry. That should tell you something. Most auto makers are
having hybrids in their ranges. Batteries have improved a hell of lot and
when production is up and prices down they will make the hybrid even more
efficient and attractive. They only way to go - in smooth style.