Changing an immersion heater is a doddle if you are confident working with electrics. First job ALWAYS is to ensure the mains supply is isolated at the consumer unit.
If the heater is mounted towards the top of the tank then you dont need to go through the rigmarole of draining the whole tank- turning off the incoming mains water then turning on all the hot taps til they stop is generally enough.
You will likely need to get the old one out first to see what length it is before buying a new one. You can get the right sized ring or box spanner from B and Q. Short sharp jerks on the spanner or a few good clouts on the spanner are generally enough to loosen it enough to start unscrewing- though it is quite easy to crease the top of the tank if the heater is mounted at the top of the tank
Read through this
http://www.ultimatehandyman.co.uk/plumb_immers ion_heater.htm
If you dont feel confident after reading that then dont!!