d9f1c7- The head gasket only needs changing if the water is turning to sludge in the radiator/header tank.
Red-berry- take the advice of TWR and give the car a good run out at least once a month if not more often, cars are designed for long journeys not short stop/start runs. It really is the best way to sludge up the engine and no special oil will prevent it, although I know a few guys at Halfords who will sell you expensive synthetic oil sating it will cure it!!
2. If you are only doing short runs blank of about a third of your radiator so the engine gets up to it's optimum temprature, check it doesn't get to hot though when you are in heavy traffic.
3. Don't use an engine flush it can do more harm than good!
The sludge is caused by the engine oil not getting hot enough often enough. It is not a problem with the oil as such though more frequent oil changes may be recommended. The cold weather we are getting at the moment doesn't help and you may well find things get back to normal once the warmer weather is with us. Bear in mind that engine oil can take up to 10 miles to get up to reach 80-90 deg C so a longer run, say once a week, may well help to keep your oil in condition.