I would use something fast draining, and slightly acid--not sure what the proper John Innes number would be. Here in the States, we generally use a mixture of all purpose potting "soil", plus sand and pumice.

I would use john innes no 2, but if they're grown in the ground they prefer a slightly acid soil, as Tugbrethil says, and as j i is not acid, perhaps an occasional feed with ericaceous fertilizer would be beneficial, Derek.