The 1.2 is quite happy cruising along a level road at 45mph in 6th; takes about 1500 rpm and could not be considered to be labouring at all.

I have a sneaking suspicion though that dropping down to 5th and thereby raising the revs slightly actually improves the mpg figure.

This is purely anecdotal and totally un-scientific as I don't really have a long enough stretch of road to check, but is the idea totally barking to start with or is it possible that with the turbo and the engine management and so on that a slightly higher rev range will actually be more fuel efficient than a lower one?

Possibly! Might be that due to the higher air pressure it has to inject less fuel, thus giving a "leaner" burn for the power?

It is quite possible. Turbocharged engines tend to have the best specific efficiency in the high torque band, so lugging it at 1500 is not necessarily the most efficient.

My experience with the diesel is that it seems happiest between 2000 and 3000 rpm, which is also where the high torque is.

It was a bit of a learnig curve coming from the PD engines, which were happest at the bottom end of this band. The CR seem to prefer it a little higher. I sometimes forget to shift into 6th and tootle aqlong in 5th without noticing. It does not seem to have any noticeable effect on economy.

...............but as you say, it is very subjective and touchy-feeley, no scientific data.

Think you may be onto something here although probably more to do with throttle opening than the engine being more efficient at higher revs. On the motorway I have driven at 60 and then 70+ but without the fuel economy showing a drop. My guess is that you need to press the throttle a little more at lower speeds to keep the speed level. At 70+ the car seems to coast along with less effort.

The turbo starts working at about 1500RPM and as turbos use waste exhaust energy to operate it would make sense that bringing it just into the turbo rev range would be more efficient.

However, only just! Don't go too far as if you rev the nuts of any turbo engine the economy will plummet, and it's not really necessary most of the time either.