I agree with Neil – remove the non-return valve and blow through with a hose pipe or deckwash pump (can be messy). Grease the impeller and ensure that all joints on the pump and hose are fully airtight. Prime the pump with the hosepipe from the bilge end and run the pump to fill it with water. Never run the pump dry for more than a few seconds despite it being "self-priming". Getting at the pump in the confined space is the worst part of the job. Success.