I'm sure it should have a nut on the top of the lid, from the picture is that a hole in the top of the lid? Oil level looks ok. Dipstick may have fallen inside the nut I'd of thought would not.
If you have a hole in top bung it up with something or nut bolt preferably. Email MMC and order a new cap. Just take a visual note of the current level and check as per normal.
Glad its finally arrrived by the way
Sorry cant check mine as its away at the moment.
