Alfadog, I was going to say what Cuzz said. There are two tappet covers: The previous generation (145/146 and 155's) had aluminium tappet covers but the newer generation Alfas have plastic tappet covers. They gave you the gasket for one of the aluminium covers. You need the gasket for the plastic tappet cover. They will rarely leak after replacement.
Try the agents, they will be more pricey but you will (most probably) get the right gasket set
(PS: I once waited 3 weeks for a gasket set for my V6 from the agents!)