You Arrive.
Locate isolation valve for cylinder and shut off.
Locate drain valve for cylinder.
A; find there isn't one and the only way to drain it is to disconnect the hot at the top and syphon the water out.
B; find it's at the back and barely accessible.
C; find it was easilly accessible but you didn't replace the washer in it when the cylinder was empty and when the water was turned back on it dripped and you had to drain again.
Start draining the water and begin to think it's taking a long time.
It's then you realise the isolation valve isn't fully shutting off, so you tighten some more and it shears closed.
Now the cylinder is empty you start to undo the heater but it's stuck firm.
You pull on the spanner harder and the boss rips out the side of the cylinder.
It's at this point you realise you should have done what EVERY plumber would have done at the time of quoting and warn that;
Even though the cylinder looks new there's a risk the heater won't unscrew or the boss may rip out.
Or because the cylinder looks old, there's a risk the boss may rip out or the cylinder fail somewhere else.
You leave at the end of a very long day with your ÂŁ90 having had to pay a plumber up ÂŁ500 out your own pocket to come and fit a new cylinder and immersion heater and or new isolation valve.
At least you can get your ÂŁ17 back from Screwfix for a low quality immersion heater.