Set Appliance State:

CH Demand
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

As the fan is running, set the boiler into diagnostic mode.
Access the diagnostic display by pressing the Mode and + buttons until the screen changes. Use the +/- buttons to scroll to 96, then hold the mode button for 5 secs.
The screen will alternately display L1 and a number (usually 0).
This indicates that you have level 1 access, the number being the diagnostic number.
Use +/- to scroll to the desired diagnostic number then press the mode button display the value.
(You can use this link to see all of the diagnostic code numbers if required)
In this instance D1 this will display the time between 2 & 60mins
Use +/- buttons to adjust to required time, then mode to save the change.