Sometimes a problem with the Rover V8. If you have a cooler, determine the direction of flow, take the INLET pipe off the cooler and insert a funnel. Fill the funnel with oil, hold it as high as possible, and rotate the engine backwards by hand. This will suck the oil into the pump.
If no cooler, pack the oil pump with vaseline. In theory you may be able to get some oil into the pump through the relief valve but I'm not familiar with this engine. I would pop the relief valve out though to make sure there is no swarf on the seat.
