Removing an old immersion heater is easy. I have removed dozens as an apprentice. All you need is a hacksaw blade with one end taped up to form a handle. Use this to saw out the fibre washer between the cylinder and the heater. You need to go all the way round and all the way through. You know you're through when you feel the blade skidding on the metal of the immersion heater. It will then unscrew quite easily. You might have to carefully file out any scores/fibre washer remnants on the flange face on the cylinder, Smear jointing compound on the new fibre washer.