I note MartinRyan's advice about the responsiveness of aluminium to heat and cold, but aluminium-bodied/framed aircraft do react a little (or in Concorde's case, qute a lot) to temperature differences. So,the only thing that springs to mind is to put it in the fridge (not the freezer) for a few hours then take it out and put it in hot water, so only the bottom of the base is immersed. After a few minutes, that may cause the base to expand a little while the lid remains cold, theoretically making it easier to free it. Mind you, if it's that old it may be time for a new one...