Checking for a return is a good habit to get into Andy , I've had no oil return on 2 occasions ,1st one about 5 yrs ago .I stopped the motor and tried all the usual things nothing ,whipped the outer timing cover off and the pump neck had split like a ripe banana (still got it as reminder to always check the tank myself for cleanliness, i've previously posted a picture on the forum ) new pump fitted and back in business. 2nd time was after a layup ( like you ) and turned out to be a non return valve which I threw out of the garage in disgust , I've only ever once not checked a return (cos we were late getting to london )and 5 miles later had a rod making a bid for freedom through the cases , lesson well learned . cheers Bob.