wee stains on mattress - how to get rid?

[shouldbedoingthehousework]

I'm just about to move ds into a full size bed and am passing his junior bed on to some friends, however the matress (which is good quality) is quite stained and looks awful . Does anyone know the best way to try to clean it up a bit (it doesn't smell or anything) Can't really understand how it's got that bad cos we've always had a plastic sheet on and ds still wears pull-ups to bed anyway

..I'm assuming it's wee stains btw - it's quite yellow...maybe sweat? Surely lo's (he's only 3) don't sweat that much anyway do they?

You can get terrible sweat stains. Speak from experience of a few years living in Cyprus. The matresses were ruined. Did he have drinks in bed? Strange with a plastic sheet on though.

He doesn't drink in bed, but can get quite hot at times so maybe it is sweat...I'm hoping it's not ruined (stains are in smallest area..) as I've promised it to a friend now