Re: Draining and old (c. 1955) immersion cylinder
If you can manage to syphon out as much as you can, then you could start to undo the cold feed with a basin placed underneath to finish the job. Actually, you can get cheap drill-powered pump in most DIY sheds these days, that with the addition a couple of lengths of garden hose should drain most of it.
There won't be a problem running the heating with the empty cylinder. You'll lose a little heat to the air in the tank, but that should be negligible.
Leo