“It is obviously desirable that a purchaser of land burdened with a restrictive covenant should be able not only to ascertain by inspection of the entries on the relevant register that the land is so burdened, but also to ascertain the land for which the benefit of the covenant was taken so that he can identify who can enforce the covenant. That latter object is achieved if the land which is intended to be benefited is defined in the instrument so as to be easily ascertainable. To require a purchaser of land burdened with a restrictive covenant, but where the land for the benefit of which the covenant was taken is not described in the instrument, to make enquiries as to what, if any, land the original covenantee retained at the time of the conveyance and what, if any, of that retained land the covenant did or might have touched and concerned would be oppressive. It must be kept in mind that, as in the present case, the time at which the enforceability of the covenant becomes an issue may be long after the date of the instrument by which it was imposed.”