Peter- great post- very useful tips... thank you.
I think I'll focus on Peter's and Dave Green's advice rather than John B's!

via an email from the Post Office... Energy Saving Trust’s top smarter driving tips include:

Drive smoothly – avoid heavy acceleration and braking.
Slow down – travelling at 70mph instead of 85mph on the motorway can save around £20 a year.
Shift up gears early, between 2,000 and 2,500rpm.
Keep tyres correctly inflated – could save around £31 a year.
Turn air conditioning off – could save around £77 a year.
Turn off your engine if you expect to be stopped for more than a minute.
Remove roof racks and roof boxes to cut drag – could save around £3 a year.
Avoid carrying dead weight – remove unnecessary clutter from your car.

Absolutely right the lower the weight the greater the fuel consumption but the size, weight and engine size of the car affects the overall saving. Best way of achieving good fuel economy is to keep an eye on the rev counter, keep to the speed limit, drive with a sensible distance between you and the car in front and look ahead. Good anticipation reduces the amount of braking, retains inertia and cuts down on fuel used. Not everyone's cup of tea but an IAM course helped me to achieve a 20% reduction in the amount of fuel I was using (also saved me money on insurance as well).

...that may be her preference John...

Steph - in future I shall drive around naked and leave my wife at home!

Since the tanker drivers strikes that caused panic buying of fuel, which in turn caused fuel shortages I've got into the habit of keeping my tank above half full- just in case there's another fuel strike.
Unfortunately this is an environemntally bad practice (shame on me) because, compared with those who run their cars fuelled between empty and half full I'm carrying an extra 25 litres. Although petrol and diesel are not as dense as water that still weighs in at about 20 kg of extra weight.
That means 20kg more to get moving every time I start or accelerate. Now if my car with me in it weights 1000kg that 20kg is 2% extra weight...so 2% less fuel economy (and 2% more carbon emissions).
Worse, if I get 70 miles to the gallon (I drive a Skoda Fabia with a new super efficient Greenline Engine) and I drive 14000 miles a year that's 200 gallons of fuel in total.... 2% of that is 4 gallons of fuel, and with fuel at over £6 a gallon that's 25 quid a year I'm wasting. (I feel ill just thinking of the money I've wasted- that's worth 8 pints of Ale, one and a half Hereford United games, or a night out for two at the cinema with popcorn!)
My advice to drive efficiently- carry nothing in your car you don't have to- extra weight costs the planet, and your pocket- as I've just demonstrated. ( Also ensure your tyres are correctly inflated as they can have a big impact on fuel efficiency too)