If the consideration, or part of the consideration, taken into account in the computation of the gain is payable by instalments over a period beginning not earlier than the time when the disposal is made, being a period exceeding 18 months, then, F1at the option of the person making the disposal, the tax on a chargeable gain accruing on the disposal may be paid by such instalments as the Board may allow over a period not exceeding 8 years and ending not later than the time at which the last of the first-mentioned instalments is payable.