Best way to clean expansion tank after head gasket failure??

[rbull51]

I have just had the head gasket repaired on my 120 but was wondering how i could clean up the expansion tank? A system flush did help to start with but the tank is getting slightly oily again from what i assume is the remains of some left over oil in the system. I was going to get a new expansion tank but want to leave it long enough for some more of the oil to come out..