Why don't you fill it through the filler hole? I'm not sure of the exact size, but you should be able to find a funnel with a long hose on it that will fit. I'd like to say the one I used was probably 1/2", or about 12-13mm. If you can't get the filler plug off, you can also find cheap plastic gear oil pumps that attach to oil bottles and have a hose about the same size. I had to get one of those to fill up an Accord's transmission, which does require filling through the level hole.
The filler plug looks like it has never been moved and is directly under metal pipe making access impossible. I know for sure the level and drain plugs were undone last year and access from the wheel arch is much easier. I have some 12mm OD plastic pipe and a funnel but want to make sure 12mm will go through the hole.
I'm 99% sure that the threaded part of the plug is wider than 12mm. More like 15mm.
However, going in from the fill plug might be easier.
The metal tube above the filler plug can be pushed slightly away from the engine. I use an 8mm socket attached to a section cut from an 8mm hex wrench. Attach a long extension to this, and the largest breaker bar you can find.
Two notes for the fill plug:
- all three plugs have blue loctite from the factory. Takes quite a bit of torque to get them out. You'll hear a sickening pop when it breaks loose. Or when your tool snaps.
- the fill plug accumulates a bit of gunk in the hex hole. Before torquing, seat the hex wrench by tapping in with a hammer. I partially stripped mine on the first attempt.