The 6 speeders i've driven top gear appears to have taken the place of overdrive, not really a gear for acceleration in most cases, offering 80mph cruising at little over 1500rpm.

Some of the Diesel offerings have such small useable power bands that to get low revved cruising speed you need that 6th...hopefully they will have lowered 1st gear on some as well for obvious reasons.

I'd guess the more gears the more likely that at least one of them is an overdrive (in the sense that maximum speed is reached in a lower gear) which allows for better economy at cruising speeds, and better official mpg. And a bit of marketing maybe.

The 7 speed DSG Popemobile doesn't appear to use 7th at all in S mode. I never use 6/6 in the Outlander when towing.

I just assumed it's for fuel efficency? no?

This may be nothing more subtle than the need to make a profit on smaller cars by fitting cheaper components. I'm no expert, but I would expect that the cost of making, say, a £10,000 VW Up is more than half the cost of making a £20,000 Golf. But they can only sell it for half as much.

Someone may have inside knowledge and can enlighten us further on this.

Toyota use a six speed gearbox on engines from their 1.33 upwards (IQ, Yaris, Auris for example), and the sixth gear is a high gear intended for economical and quieter cruising.

As regards production costs of models like the Up/Citigo/Mio, and the Aygo/C1/108, these are reduced by basing production in countries like Slovakia (Up/Citigo/Mio) and the Czech Republic (Aygo/C1/108), where labour rates are less. Another example of this is the current Seat Toledo which is made by Skoda alongside the Rapide, and the forthcoming Seat SUV which will be made by Skoda as well - in the Czech Republic.

My last 3 turbodiesels have had 6-speed manual boxes, with varying degrees of success in terms of drivability and economy.

In the Passat B5.5 PD130, the ratios were quite closely stacked to fully exploit the 'torque band,' although the engine was very flexible and didn't really need 6 gears. However, It did mean that 4th gear was shorter than in the 5-speed gearbox variants, making it great for single-carriageway use, as at any speed above 35mph you had big overtaking grunt instantly available without needing to change gear. 5th was useful for long-striding economy on dual carriageways and 6th reserved for motorways.

In the Mondeo IV 2.0TDCI, the gears were too high for the engine's torque and car's weight, so 6th was very much an overdrive and only really usable at the legal limit on motorways. This dented economy, too, unless you drove like a granny.

In my current Volvo V60, 4th, 5th and 6th are very usable thanks to the engine's high torque output (295ft-lb from 1,500 to 2,750rpm). Although 6th is very tall (at 70mph, the engine's turning at 1,800rpm) there's still plenty of grunt while enabling 50+mpg economy.