Bingo!!NO - thats not it!
If you look at the pic I posted before you will see part of the same splined plug (around the fat red battery cable) in addition to the normal square hole filler plug further away - do not remove that - use the correct plug requiring the square driver.
Looks like you do have a DP0 there ok from what I have as reference material - model FC0L
If you compare the pic I posted with the square hole to the last one of yours, you will find the filler plug in the location I circled in red in the dark there.
View attachment 179683
Once part of the air intake assembly is removed (it easily unclips), the fill plug is visible and easy to access. A photo is attached. I need to get hold of a funnel with a long flexible spout and a narrow spout opening. These are readily available from the local equivalent of Halfords.
With luck I can get the job done this weekend and will let you know how I get on.