It smells bad, and there's some kind of film (bacteria?) on the top of the water. This is a loo which doesn't get used very often, which may have something to do with it.
Not sure how to approach this. How to get 100% of the water out? It doesn't all disappear when flushing, so flushing with the water switched off wouldn't do it. If that's not possible, how to clean it out properly? Would prefer not to have to put some product in it which will only make the cistern smell strongly of perfume or itself, and would rather end up with clean water after cleaning out the thing itself.
I've drained a cistern in the past by turning off water, flushing,scooping out what I can with a small cup then finally using a sponge to absorb the rest of the water by dabbing/wringing.
I,d then use bleach in cistern to kill bacteria, spray bottle would be best to cover walls and workings - leave to work for 30 mins before turning water back on and flushing a number of times to rinse bleach/scum.
I'd then drain cistern again and look at getting rid of hard water deposits as the minerals in them feed bacteria. Either use viakal or White vinegar(preferably left overnight) before refilling/flushing/rinsing again.
try a Parazone Bleach Cistern Block. As well as killing mould and slime, it will slowly dissolve limescale.
One thing to look out for, bleach does shorten the life of rubber seals.
If the incoming water supply to the ballcock comes in through the side of the cistern, it is above water level and not affected. However if it comes through the bottom of the cistern, it is possible for the joint to eventually drip.
I don't think I've seen it happen on cisterns less than 20 years old.