best to mix your own or at least cut the ready compost with some perlite.
try mixing 2 parts loam/john innes/topsoil with 2 parts peat/coco and 1 part perlite
even better if you can add 1 part mushroom compost or worm caste
i imagine the westland+john innes was to hot (nutrient rich) and wet for the cheese.
i like to buy unfertilised stuff - peat, topsoil - (westland is ok) and then add my own organic ferts and powdered dolomite lime . look in the organic section for soil mixes