Could be the fuel pump gasket which has failed. Try getting underneath and wiping the crankcase clean below the pump, then check again in a day or so. This should let you know if it is the pump gasket or not. If it is, then you can buy a gasket set fom the main agents or an accessory shop if you feel confident to fit it - not too difficult. But you could just try tightening the pump bolts first - there are usually three of them. I've had this problem on several cars before.