Thank you all AGAIN! More progress below. I'm 95% there. See new results from 6. onwards. Basically, removing as much water as you can (with a little washing powder scoop) gets best results (but what an interestingly foul smell!).
1. Gumption, Jif, Domestos - didn't work.
2. A cup of usual washing powder left to soak overnight/24 hours - got rid of heaps but not all.
3. McKenzie's Citric Acid (the 75 gm little container from the cake aisle, $2.30 Woolworths) added to bowl water, stirred to dissolve, left to soak 15 hours - got rid of some parts beautifully, but left stubborn stains down each side at the bottom where you can hardly see and some in the middle. Maybe I should have taken out more water and used bulk citric acid?? Hopefully the word "acid" doesn't mean it's giving the toilet bowl cancer or something.
4. Half a cup of vinegar in bowl water with generous dollop of Bicarb Soda, scrubbed with toilet brush and flushed immediately - no fizz and didn't work.
5. Used cup to get rid of lots of water then added vinegar and bicarb soda again, big fizz, left 10 hours, scrubbed with yellow sponge/green scourer thing, added more vinegar and bicarb to scourer and scrubbed away - removed even more, but still those stubborn stains remain!
6. TOOK OUT LOTS OF WATER! (Ugh - foul smell) Tried leaving bicarb of soda paste on for couple of hours, then scrubbing – didn’t really work.
7. TOOK OUT LOTS OF WATER Threw in vinegar and a heap of bicarb and soaked overnight, then scrubbed – made a significant difference!!!!! Now there are only brown/faded to grey stains left down the bottom on the sides, almost out of sight. Near enough is good enough. But it isn't!
I’m determined to keep experimenting until I get rid of the last bit. I might give the Alka-Seltzer a go, or more citric acid in bigger concentrate this time, or another soaking of fizzy vinegar/bicarb. I haven't tried the CLR, but someone else did and only achieved 40% result. Whatever I do, I’ll scoop out as much water as I can.