Immersion Heater removal - stainless tank

[NEC]

Hi All

First post and wanting to know how I may get on taking a 3kw Immersion Heater out of a Heatrae Sadia stainless steel tank. It has been in situ for over 15 years and initial attempts with a full tank of hot water have failed - it appears to be seized solid.
I recall taking one out of the old copper cylinder and 'scrunching' up the copper - I don't want to do this to this tank.
Am I being overly cautious in that these tanks will stand any amount of pressure they being steel? Anyone with the experience that can offer some advice would be greatly appreciated.
My only other option is to transform the power down - I want to use the spare Solar PV generated during the mid day.