Firstly you shouldn't be touching it unless you have G3 qualification aka unvented ticket. As a certain Doctor Drivel on here keeps reminding us, it is potentially a bomb in the hands of the untrained.
www.waterheaterblast.com shows a 54 litre tank that to be fair has been messed about with so that it explodes.
Secondly I can tell from your post that you are somewhat experienced and will probably do the job anyway. The least I can do is to show you the instructions and stress that you adhere to them carefully. Also you will need the correct tool for removing and replacing the immersion heater which should have been left with the instructions or may be secreted around the cylinder somewhere. If not you will have to get one from heatrae sadia.
The instructions are
HERE I know you are not are buffoon but I must stress the importance of correct wiring and that you must not by-pass any stats