You should be able to connect it at the local switch in the airing cupboard. If there is no switch, or the switch is very old and unsound, come back and there are a couple of alternative ways of doing it safely. Do not make a "temporary" joint in the cable, there are correct fittings available for only a few pounds depending what you have got.
What other ways are there of connecting/suppling power to an immersion heater, out of interest?...
What other ways are there of connecting/suppling power to an immersion heater, out of interest?...