Cif, used to be Jif, and a scouring pad or sponge.
Takes a bit of elbow grease but never had better results with anything else.
Once it’s nice and clean give it a coat of Fabsil or even Thompson’s water seal.
As Wiggy says, that cif is pretty good stuff, cillit bang is another. I’ve had great results by using genuine fairy liquid and letting it soak then giving it a scrub.
Ozziedog,,,,,,,,,trying different things can help .