Panic - dirty loo!

Try rubbing alcohol mixed with bicarbonate of soda. Leave on as long as possible then toothbrush to remove.

Spirits of Salts from the hardware shop. Be very careful with it, but it will shift absolutely anything. Hydrochloric acid basically, so the limescale which is holding the stain will vanish away!

I would think scouring could damage the surface of the ceramics and cause more problems long term. You need a mild acid to clean lime scale.

Just looked up in How Clean is Your House and they recommend baling out the water (use a big car sponge and rubber glovers, squeeze out into a bucket!) and then covering the limescale/stains with a thick paste of laundry borax and vinegar, leave on for a couple of hours. Probably need to use a good scouring pad on it after this.
I had never heard of baling out the water, but had to do this once when cleaning the loo of a house we rented out. I think in hard water areas, the scale makes staining so much worse.
Good luck!

I just read that WD40 gets rid of old stains in toilet bowls, but only use a little:

lifehacker.com/5918339/use-wd-40-to-break-down-tough-toilet-stains

What about old-fashioned washing soda?

It depends if it's stains or if it's limescale in which case you need a limescale remover.
Friends used caustic soda on theirs, which worked.

I recommend white vinegar. Use a loo brush to push down as much of the water in the bowl as possible (to avoid diluting the vinegar too much)and then tip in white vinegar up to the usual level. Worked a treat on my 'hard to shift' stains which had collected on the corners.
The vinegar I got was 'Stardrops White Vinegar spray' and was £1 for 750ml in Boyes (a well known North Yorkshire shop).

Try large bottle of Coke overnight.

Spirits of salts is Hydrocloric acid. Lot's of patio cleaners are also HCL

You do need to use it with care and keep it well away from caustic soda.

DD had the same problem in a rental house. I got it off using a heavy duty descaler bought on Ebay. Unfortunately that won't help your current dilemma with visitors, but may be worth trying in the future.

When I trained as an Infection Control Nurse we were taught that bleach makes porcelain on older loos porous over time and the stains and the bacteria then are very difficult if not impossible to remove hence why we didn't use bleach in hospital loos. You may have to have them replaced at some stage.
N.B. Spirit of salts is very corrosive so do be careful how you handle it.

there was a disgusting video on utube recently where they poured white vinegar down a revolting toilet and claimed it cleaned it after leaving it to soak

If coke fails, get to a plumbers trade shop and get the professional stuff.

There used to be a chemical called I think, Spirits of salts. We used it years ago when we had a similar problem.

Good hardware shops used to sell it. DIY like Wickes might.

x

Read somewhere if you tip can of Coca Cola down and leave it overnight
it removes stains ( hope you don't get took short during night also has put me off Coke )

You could put one of those blue capsules in the cistern which will disguise it if all else fails.

If bleach and scrubbing haven't worked, there doesn't seem to be much you can do! Our upstairs loo is an ancient one with a high cistern and pull-chain and the bowl is crazed and stains easily, but bleach works OK.

Fortunately most guests use the modern downstairs one!