Depending on the type of gasoline or, as ya'll call it, petrol, water can be a problem. For example, in aviation grade fuel (100LL) water is heavier than the fuel. In the winter, as jtp indicates, the water can freeze. However, being heavier than the fuel, it forms ice at the bottom of the fuel tank. In smaller aircraft, there are quick drains installed in the tanks that can be sampled to see if there's any water. If frozen the drains may be forced open, but will remain open and leak fuel. If it's really cold, say 10 degrees F or lower, it's advisable not to attempt to open the drains since the ice won't enter the engine laying, as it does, on the bottom of the tank. Jet fuel, however (it's nearly the same as Number one diesel fuel) is almost the same weight by volume as water and water tends to remain suspended in the fuel. Since it can't be drained off as can aviation gas, additives are placed in the fuel, such as Prist to alleviate any problems with ice crystals in the fuel lines... For high altitude capable aircraft (such as the usual passenger jet) the jet fuel is actually heated before it enters the turbine by an engine oil to fuel heat exchanger...pedantic, no?