Compensation on Rovers and Rangers is calculated in two steps.
Step 1: Look at the Anytime Day Return (or equivalent) for the journey.
In this case, there are only Anytime and Off-Peak Singles, so you'd use 2x the Anytime Single fare of £43.15 (after discount) as the basis, for a notional Anytime Day Return of £86.30
For a 40 minute delay, the base compensation is 25% of the Anytime Day Return, which is £21.575 in this case.
Step 2: Calculate the notional value of a Single journey on the Rover.
This is calculated in the same way as for a season ticket, so in this case the ticket is nominally valid for 8 Single journeys. This gives a nominal value of £10.56 (rounded to the nearest 1p.)
For a 40 minute delay, you take 50% of that value, giving £5.28 (rounded to the nearest 1p) as the compensation due.