Don't call an electrician as many of them don't have the special spanner which is needed to remove and replace an immersion heater. They'll have to go to a hire shop to get one and they'll probably charge you a lot extra for doing so.
If you want the job done cheaply, ask around for the name of a good PLUMBER.
Alternatively, as long as you hire the necessary spanner, it's a really easy job to replace an immersion heater. (You can buy a replacement immersion heater at any plumbing store). Simply turn the power off at the mains. Disconnect the leads from the old heater. Unscrew it. Screw the new one into place. Reconnect the leads and turn the power back on. (It might be necessary to partially drain the tank first, by turning the water off at the stop tap and then opening a hot water tank. If so, remember to let the tank fill up again before switching on the immersion heater).
Chris