PinkCosmos
Thank you everyone for your advice.
I think I will give them all a go apart from the washing machine suggestion. They are leather with an imitation cork sole and suede foot bed - see picture. I think they would be ruined in the machine.
They assuredly would be ruined in the washing-machine.
The stuff we used on tennis shoes was called wet white - at least in the Navy where it was used for cleaning uniform shoes in the tropics. I doubt you can still get it, and it certainly was not recommended for leather,
Have you tried ordinary white shoe polish? This may be a stupid question, but you didn't say you had.
If you had and it didn't work, a shoe-shop or dept. in a department store that sells your sandals ought to be able to recommend a product.