Basically an immersion heater has an on off switch which can ba a fused spur unit if wired into the ring main or a on off switch with no fuse and having its own fuse/contact breaker at the fusebox/consumer unit. In either case it can also have a timer which can be either a digital or electro mechanical, but then again could be controlled by a timer that is the property of the electicity suppliers. In all cases it will have a thermostat that is accessible underneath the removable top (normally secured by a slotted screw or 8mm af nut) this is traditionally adjustable but over the last 3 or 4 years has been factory set to approx 65C and with a high limit cut off, before that the `stat was adjustable but ideaaly should not be higher than 60/65C as apart from the scalding aspect, limescale is deposited alot more at or above those temperatures - in other words boil the kettle for a bath

But seriously if you turn the immersion on in theory it should regulate itself through the thermostat but depending on your experience in these matters you could check the `stat setting by removing the imm htr cover (while switched off) or failing that ask a qualified plumber or electrician to check it for you...

bet you wished you`d never asked!