If you look at the full manual the last page p40 it shows the backbox flashers wiring, they are type 89 globes and are driven by a solenoid. (Edit: I should have said 'driven the same way as a solenoid') The instruction booklet says they are driven by solenoid number 15, so you can use the solenoid test function to test them.

Check your fuse rating, the standard WMS solenoid fuse F2 is 2.5A slo blow, shown on the power supply schematic.

Also check the resistors shown in the backbox for the flashers - these have often suffered due to heating over the years. I upgrade the 330 Ohm one to a 5W ceramic and replace the 2.2 ohm one if it looks stressed.