We have a similar issue, as in the only thing you can put down our loos is paper - we’re in the “old town” and the sewage infrastructure here can just about deal with paper, hence a bin in every loo. And a plunger in the cupboard!
My OH uses Vanish, which I think is the european equivalent to Oxyclean. Soak them for 15mins and if that doesn’t work a mild solution of bleach. No idea about a hot or cold wash, I must ask her!!!
JF