Rover 100 got emulsion of oil and water in engine,

possibly head gasket but don't rush into an expensive fix too soon. if you only do regular short trips i.e 1 mile to school then back again later in the day , its normal to see emulsification in the filler cap as the engine runs cold with no chance to heat up properly and evaporate off natural condensation. I'd pull out the dip stick to see if the oil in sump is mixing with oil - if so almost certain to be head gasket. if not check your coolant level maybe you have a leak such as burst hose, leak from water pump. if your not sure get to a reputable independent garage and explain the problem to them maybe ask them to carry out a combustion leak test , that should confirm or otherwise the head gasket situation. sometimes called a block test. could be the best £30 you'll spend.