It's off!!!! Whoohoooo. The funny thing is, I think it was a bit of a combination of several of your methods which did the trick. I tried the hitting gently with a bit of wood, and that seemed to very slightly prise it open on one side, and in fact some of the soapy water came out of the joint. Then I tried heating the lid again with cold water inside. We went out for several hours and when I got home I picked up the tank. As soon as I touched the lid it almost slid off. Thanks all.