Try Soduim metabisulphate.You may need to do 2 or 3 mild applications but you will find that it will reduce it completly.
I now had just managed to get the formulas right for tea and coffee tannin.
I just finished a expesive green soft wool chinese rug that was covered in tea stains as a full pot had dropped on to it.It had been cleaned prieviosly ( by a well known franchise
) and was told that the stains where set.
I was reccomended as I had done a similar one ( tea stain removal) for a friend of his.
I had the rug for 3 weeks it took 6 applications but it now looks like new.
Rug cost £1250
My fee £125 ( 10% of value) & got £25 tip.
And a very very happy customer.