Release two pieces of silver stuck together?

I have a customer who has an OLD English glass and silver flask.
The silver top is on a hinge that comes down over the top of the
glass flask and then turns with an interlocking arrangement on to a
silver collar. The silver top has a cork insert to make a complete
seal and is made up of two pieces, one that turns inside of the
other. However the two pieces do not turn as they are stuck
(corroded or ?) tight. How can I release them without applying heat
because of the cork? All suggestions gladly received.

Thanks
Steve

However  the two pieces do not turn as they are stuck (corroded or
?)  tight.  

Have you tried WD-40? Or Silicone?