That 'coffee cream' stuff in/around the oil filler cap is a sign of a blown cylinder head gasket. It is an emulsion formed by pressurised water being forced into an oilway. You might also find an oily film floating on top of the coolant water if you remove the radiator cap - due to oil getting into the coolant. Replacing a cylinder head gasket is an easy enough job for a weekend provided you have a garage to work in or wait untill the weather is better. A new gasket set will set you back about thirty quid or so and you'll need to add a torque wrench to your toolkit if you don't already have one. Depending on the engine's mileage, it might be worth doing a valve grind as well just to keep things sweet. Even the Haynes manual is good enough to see you through a simple head gasket change, but the proper Rover manual would be better.