“… the question whether the [Professional Conduct Committee] should exercise its rule 51 power to refer the case to the Health Committee should be considered in conjunction with the question whether the case is or may be one which calls for a direction of erasure. The [Professional Conduct Committee] should not, in their Lordships view, refer a case to the Health Committee unless and until satisfied that a direction of erasure would not be the right direction to make. And once the [Professional Conduct Committee] has decided that a direction of erasure is the right direction to make, the question whether the case should be referred to the Health Committee has received its answer.”