Please help! Mystery stain on cream jumper.

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I'm baffled! I bought this silk/cashmere jumper shop.mango.com/AM/p0/mango/clothing/cardigans-and-sweaters/cashmere--silk/silk-and-cashmere-sweater/?id=73210252_B2&n=1&s=prendas&ie=0&m=&ts=1356232137935 which I love. I washed it, put it in the airing cupboard and when I got it out it had a mystery stain on the front. At first I thought it might have been a dye transfer from new jeans I'd washed, but the colour isn't blue, it's just slightly darker than the cream.

Anyway, I hand washed it again and cautiously dabbed some oxy stain removing crystals on the mark (it says not to use on delicate fabrics) but it's made no difference. Is there anything I can do to remove the stain without shredding the jumper? White vinegar? Vanish?

Thanks in advance for any help you can give - I'm dreadful at washing.

Could you have spilt some kind of fruit juice on it? Pear, apple or plum juice doesn't necessarily show up when you spill it, so you don't realise it's there. Then when you wash it, the hot water sets the stain and you're lumbered with it!

Sorry not to be more help - if this is the case the only way I've found to get rid of the stain is to hang the offending garment in the sun, which bleaches it out. Once it's set in, you've had it.

Someone on a Credit Crunch thread I'm on says she only ever uses washing up liquid on stains, maybe that would work on this? I'm trying it today on a sweater, but mine isn't cashmere. Maybe try it on a less obvious area to check colour fastness first?