Oh dear ..... this is why you're just a spark (allegedly) & not a plumber !!
NEVER try to loosen an immersion heater with the cylinder empty of water - you'll knacker the top of the cylinder - EVERY TIME!!
So, with it full of water, run you hacksaw blade between the flange on the top of the cylinder & the underside of the immersion heater to remove the gasket & reduce the "tightness" between immersion heater & cylinder THEN loosen the immersion heater by sharply tapping the big hexagon nut with a hammer & drift, then putting you box spanner on it & sharply tapping that with your hammer.
Once you have it loosened THEN drain SOME of the water from the cylinder so that you DON'T have a flood!!
I cannot believe that we have 26 posts spread over two pages for such a SIMPLE job !!
Jesus Christ man ... shouldn't you be stacking shelves in Tesco???