The joys of fuel consumption - impossible to compare from person to person really.
Big factors that will affect it:
Weight. If the estate is heavier than the saloon the urban cycle (town driving) will be affected.
Aerodynamics. Saloons tend to have a lower Cd than estates due to the tail being stepped off more gradually, and if the estate has roof rails that will make a difference to the frontal area. Wide tyres increase frontal area too.
Rolling resistance. Underinflated tyres turn movement into heat, sticking brakes etc make a big difference. Try running with tyres at the max recommended inflation (note your roadholding will be affected...)
Winter. Winter diesel tends to have more additives and slightly lower power and therefore you use more. Also your cabin heater (fuel driven) will be in use more and for longer so the start of your journey will use a lot more fuel until the engine warms off - on the 211 you turn it off by disabling the aircon, not sure about the 210, but it saves a lot of fuel.
Driving style. The more you brake, the more you have to accellerate again and the more fuel you will use - anticipation and smoothness is the key.
I drive fast and quite aggressively (not in my opinion but normally that of the white knuckled, pale and trembling passengers) and average 32mpg ish in my 211, days when I'm doing town traffic only are 24-25mpg, motorway is 34-38, and when I'm deliberately being laid back 40-45.
To compare, my A class averaged 38mpg, my Volvo S60 2.0T 22-24mpg.
I expect there's nothing actually wrong with your 210, you'll find a taxi driver will not use the aircon, will have his gearbox trained to economy (drive gently for a week, feel how sluggish it will become and how early it will change up!) and will probably run high tyre pressures and small tyres for cost reasons. Also his being a saloon it's more aerodynamic and probably lighter.
