I have found some mould on my mattress where it sits on the slatted base. Luckily, I have one of these Ikea mattresses where the cover comes off the foam bit IFYWIM. The foam bit was clean, the only stains are on the covers. I have soaked it in Milton and scrubbed it and the stains have disappeared. It's now drying in the sun. I will take it to a laundrette tomorrow and wash it properly.
I know once mould is there it is very likely to come back but would a mattress cover help prevent it or would it make things worse?