There must be a way to drain it, because i doubt the heater is the original
I would just try the easy method first. Tie the ballcock up, then turn on all hot taps to drain. See if it works.
please for the love of god stop suggesting this it will not work if the cylinder has been installed correctly it will not drain.
Ok then smartarse, if you're the expert then why not suggest something to help him instead of calling everyone noobs because we aren't plumbers?

Sounds like it's done already but if the issue was just finding a drain point then why didn't he just unscrew the cold feed coming into the tank and the water would have drained out from that.
normally you just siphen out the water usning a hose, to outside or down the toiler, you just suck the water through the hose and leave it, doing its job

That would work is the immersion heater was at the top, which it was not. It was at the bottom. You open that, and lo and behold comes a torrent of water everywhere.
was their nowhere ontop of the tank where you could slip a hose in ?