Petrol engines work by mixing air with the petrol. (Diesels don't mix air with the fuel, so that's why it was important to know which type of engine you've got). If the mix is wrong, or the spark that causes the mixture to explode doesn't occur at the right time, the fuel is burnt inefficiently, resulting in poor emission figures.
So the first thing that any garage should check is that the air intake isn't blocked. (A simple thing like a piece of litter getting caught over the air intake could be the cause of the problem). Then they should check the air filter. (If it's just dirty it simply needs changing but if it's clogged up because it's covered in oil then the source of the oil leak would need to be checked). Next they should check the timing of the engine, to ensure that the spark is occurring at the right time.
All of the above are simple checks and, if a fault is found, usually simple to fix. After that though, as Bright Spark has suggested, far more expensive things have to be examined. (Catalytic converters for Range Rovers don't come cheap!)