Hello Tor,
There are a variety of problems which can contribute to poor fuel consumption. As an idea, worn timing chain and camshaft,..dirty air filters, idle mixture setting too rich, float levels more than specified, less ignition timing than specified and one of the biggest causes of poor fuel consumption,...a vacuum advance module that is no longer working. Usually the diaphragm within is perished, but sometimes the port on the carburettor and / or the hose linking the module to the port is blocked.
How many miles / kilometers has your Dad's Rover covered?
Ron.