As part of "Operation sort out the screw ups", aka correcting what the people who were in before me did, and a general renovation, the immersion cylinder has been removed.
It's currently propped over the bath, having been left to drain out the last of the muck. After draining, via the, painted over, drain c*ck, I poured what was left in to the bath. Urgh! Yellow and very cloudy, nice. Along with some nice lumps of blue/green stuff, which I assume is limescale.
So, questions, how to remove this manky muck? Stop up the bottom hole and fill with water and descaler, leave, drain and flush?
Any better suggestions, a new tank is 100ukp from Screwfix, so that is an option.