To be fair, according to that document, the tax relief existed before then, it was changed to "at source" in 83.
"Mortgage interest relief was introduced in 1969 to give relief for the interest paid on
loans for the purchase or improvement of land. These categories of interest relief
were subsequently expanded to include the relief for interest on loans to purchase life
annuities where the annuitants were over 65.
Initially relief was given for interest on the full amount of a loan of any size. The relief
was subsequently restricted and is now limited to interest on £30,000 of a loan.
Before April 1983, the allowable interest was deducted from taxable income, either in
an assessment or by increasing the PAYE (Pay As You Earn) code number. In April
1983 the mortgage interest relief at source scheme (MIRAS) was introduced under
which the borrower pays the lender the interest less the tax relief."
So gant may have had a mortgage with the interest relief in 79, but it wouldn't have been MIRAS as the "at source" part of that wasn't around then.