Fairplay, you ask if the interest received at end of year 4 could put your relations above the Dirt exemption level, and the answer is yes it could. The interest is taxable income at the date of receipt.
The other thing to think about is that the higher tax-exemption level for over 65s is under notice. Two or three years ago at Budget time it was stated this higher exemption would be removed in 4 years. It has already been reduced.
In these times of uncertainty and mini-budgets, they should take their interest as soon as possible. Even with a one year deposit it could be wise to take the interest monthly.