There was a nice long thread about exactly such a thing a few weeks ago. I remember replying to it many times.
You might want to call a plumber in. When replaces it, watch him and learn. Next time it goes, you do it yourself.
I know that Toolstation have replacement heating elements and thermostats for around £15. You will also need an immersion spanner, which would set you back around £10.
With regards to draining that tank, thats childsplay and costs nothing.