(1) Where -
(a) a scheme which is or has at any time been an approved scheme, or
(b) a statutory scheme established under a public statute,
contains a rule allowing in special circumstances a payment in commutation of an employee's entire pension, and any pension is commuted, whether wholly or not, under the rule, tax shall be charged on the amount by which the sum receivable exceeds -
(i) the largest sum which would have been receivable in commutation of any part of the pension if the scheme had contained a rule providing that the aggregate value of the relevant benefits payable to an employee on or after retirement, excluding any pension which was not commutable, should not exceed three-eightieths of the employee's final remuneration for each year of service up to a maximum of 40 years, or
(ii) the largest sum which would have been receivable in commutation of any part of the pension under any rule of the scheme authorising the commutation of part (but not the whole) of the pension,
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