The comprehensive agreement (SI1984/133) entered into force on 20 January 1984 and has effect
a) In the United Kingdom for
i) Income Tax from 1984-85,
ii) Capital Gains Tax from 1984-85,
iii) Corporation Tax from 1 April 1984.
b) In Tunisia for
i) Taxes withheld at source on income paid or credited from 1 January 1984,
ii) Taxes on other income for taxable periods beginning on or after 1 January 1984.
The text of the Tax Treaty can be found via https://www.gov.uk/government/publications/tunisia-tax-treaties