Where the consideration or part of the consideration taken into account in the computation of a chargeable 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, if the person making the disposal satisfies the Revenue Commissioners that such person would otherwise suffer undue hardship, the capital gains tax or corporation tax, as the case may be, on such a chargeable gain accruing on a disposal may, at such person's option, be paid by such instalments as the Revenue Commissioners may allow over a period not exceeding 5 years and ending not later than the time at which the last of the first-mentioned instalments is payable.