I did this two weeks ago.... Here's what I did:
1. Full Ring spanner (not the half, its a waste of time)
2. a ball of Hemp
3. Tub of boss white (or similar jointing compound)
Before unwiring the old immersion take a photo of the wiring into the switch (or make a wiring plan).
Removing the old immersion:
Do as above (drain tank, shut off feed etc). Have plenty of towels handy in case you didn't drain enough (I was in a hurry and flooded the feckin kitchen ceiling
Once the old immersion is out, clean out the fitting thoroughly and dry it.
Inserting the new immersion:
follow the instructions on the boss white tub: basically strand the hemp into the grooves of the screwplate of the new immersion towards the top, using hemp: layer of Boss White.. and finish with a layer of boss white...
Put the immersion into the tank and, making sure the thread wasn't crossed, screwed the unit back gently, tighten until firm (but not overtight).
Refill the tank and turn on the cold feed, check for leaks..
I then put the heating on to bring the tank back to pressure to see if any leaks.
Once I was happy, I wired the new one up.
Then it still wouldn't work, so I got an electrician out who told me the switch was gone and I did'nt need to replace the immersion in the first place
no sign it had gone tho, light's still on, everything...
so, check the switch first eh... cost me the bones of 120 euro in the end.. all for the price of a switch (20)..