The comprehensive agreement (SI1970/153) entered into force on 5 February 1970 and has effect
a) In the United Kingdom for
i) Income Tax from 1968-69,
ii) Capital Gains Tax from 1968-69,
iii) Corporation Tax from 1 April 1968.
b) In Finland for Finnish taxes for any year of assessment beginning on or after 1 January 1969 and chargeable on the income or capital of the tax year 1968 or thereafter.
Further agreements (SI 1973/1327 and SI1980/710) amending the provisions of the 1970 agreement relating to dividends and to tax credit relief in respect of dividends entered into force on 7 July 1974 and 25 April 1981 respectively. The second amending agreement had effect in relation to dividends paid on or after 6 April 1975.
The text of the Tax Treaty can be found via https://www.gov.uk/government/publications/finland-tax-treaties