From the photo the test is good to 2017, then there's 1 1/2 years to the final date.
So 'toms magic depritiation formula' valuation is:
If the buyer doesn't value doing a test for the final life:
Total life cost £200 / 15 * 4 years = £53
If the buyer does feel testing is worthwhile then:
£200 / 15 * 5.5 then deduct a test fee:
£73-£25 = £48
(It becomes worth less as you don't get the full value of a retest due to the split timing)
** of course a deduction could be made for second hand value, and an addition made for anything special about the cylinder ~ and no-one is bound by my formula **