Round the gasket Dave. They never seal properly as I found out on my old NAD. There's not enough torque on the bolts to make the difference. It's the same principle as your axles, dump it in cold water and it sucks air (or water) through any available weak point.
Someone somewhere does a breather kit with a plug and tube, the same as an axle casing. The only draw back to these are should you have an oil leak anywhere behind your timing casing then the oil can't escape and not only fills the timing case but blocks the breather pipe.
